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Having one big data file and the OS caching it are two different things. Most of the comments here seem to assume that the app, in this case Outlook, orders the OS to cache the datafile while really it's the Sysmain background process (Superfetch, whatever you want to call it) that will cache it if the file is accessed frequently enough.

Though not publicly documented, the PST file format is said to be designed around a relational database (Source: Ontrack). Strangely, the PST/OST file format is not on the MS Open Specification Promise.
 
Now we're done with the facts, and I'll play the devil's advocate now. That it's one big file must have something to do with data consistency. Otoh, caching the file can be advantageous because querying a relational database from RAM is really fast, even without full-text indexing, and I don't see why not if you have 400 MB to spare. Searching a random word, time range, attachment name, etc. in all of your mail can be completed near instantly.
Sure, a structured... thing... on the filesystem works fine for many applications, but in that same 400 MB store, a huge folder tree with files really isn't the fastest way to access (or more important, search) data.
I personally like large files for data stores because it is easier to make a consistent backup of and it generally prevents mucking about with the filesystem.

http://www.ontrackdatarecovery.com/ probably one of the best corp in their section... good app choise and also services where you are sending them your hard drive and letting them to do the hard work(only when the hard drive is very dead).

I used to be a software engineer for Ontrack Data Recovery, one of the major data recovery companies. Perhaps not surprisingly, our data recovery tools were proprietary tools custom-written in-house. It's not something that was available to, or marketed to, the average Joe (or even the average Joe Programmer).

Ok, I took at look here at the sort of data loss they work with. They mention logical failures and mechanical failures. They don't mention deliberate overwriting. Everything on this page discusses just how hard it is to find the bit you're looking for, and how Ontrack has all sorts of expertise in coaxing the drive to do what they asked to at least get something out of it.

So far I haven't seen anything to suggest they can recover deliberately overwritten data. In fact, their data analysis page says:

Although electronic evidence is especially fragile - prone to erasure, destruction and tampering

Everything on the site points to being able to recover files that are inaccessible either due to drive failure (including mechanical damage) or being deleted without being overwritten. If you can find somewhere where they claim that they can recover overwritten data (as opposed to merely inaccessible data), I'd love to see it. Otherwise, you haven't disproved the notion that recovering overwritten data on a modern drive is an urban legend.

Ok, I took at look here at the sort of data loss they work with. They mention logical failures and mechanical failures. They don't mention deliberate overwriting. Everything on this page discusses just how hard it is to find the bit you're looking for, and how Ontrack has all sorts of expertise in coaxing the drive to do what they asked to at least get something out of it.

So far I haven't seen anything to suggest they can recover deliberately overwritten data. In fact, their data analysis page says:

Although electronic evidence is especially fragile - prone to erasure, destruction and tampering

Everything on the site points to being able to recover files that are inaccessible either due to drive failure (including mechanical damage) or being deleted without being overwritten. If you can find somewhere where they claim that they can recover overwritten data (as opposed to merely inaccessible data), I'd love to see it. Otherwise, you haven't disproved the notion that recovering overwritten data on a modern drive is an urban legend.

Searching through their web site, I do not see anywhere where they claim to be able to recover data from files overwritten on a hard drive. For instance, the information available here seems to suggest that they correct mechanical problems with the drive, and then copy the drive image and work from the data there. That procedure will work for damaged drives or non-overwritten data, but it is not going to help if the data on the drive was overwritten.

Their part in recovering data from the hard drive from the Columbia is impressive -- there is a description here -- but it did not involve recovering overwritten data, and thus is no evidence in support of them being able to do so.

If this company is really capable of recovering data that was overwritten, then why don't they seem to advertise the fact anywhere?

This has been known for many years. Here are more taken from my personal ant Web site:

Ants in yer... Pants? NOT! (Toshiba notebook/laptop); Ants Invade Apple iBook.

Ants In
My Nokia Mobile Phone (A Yahoo! account is required).

Ants in Omniview switchboxes: An e-mail story of ants invading a network
switchbox. Thanks nTrFace.

Argentine ants invade a network hub.

Ants had taken up residence in a guy's external hard drive: Ontrack
and Computerworld
(seen on /.).

A photograph showing ants nesting in a guy's phone box, affecting his DSL connection and phone system.

get an identical drive; put the old platter assy into the new drive's guts, or just move the good drive's electronics over

That's the hard part. "Identical" means not only model, but often revision as well. Once I did get lucky and find another drive from the same batch, and successfully trade circuit boards. But a couple of other times I failed to find the same rev. number, and the transplant didn't work.

I've been successful a few times freezing the drive (sometimes extending runtime with a can of freeze spray, an aerosol like canned air but gets a lot colder, intended to help techs find thermal problems). And mechanically abusing it (twisting it to start the platter spinning, or just whacking it.

Always have everything ready to go, if you do get it started it may work for ten minutes and quit. Maybe you'll get it started a second time, maybe not.

When the problem has (apparently) been data corruption rather than a hardware problem, I've been successful with software a few times. Once with OnTrack EasyRecovery, several times with File Scavenger. Including once where the problem was obviously a head crash, the drive made a horrid screeching sound. Couldn't get all of the data, but got some of it.

There's a pdf at http://www.hddrecovery.com.au/ that's got some other suggestions. (I have never tried that company's recovery software so have no opinion on it.)

I've never had anybody who was willing to pay to have the data recovery pros do it. But often they'd be willing to go a few hundred bucks for me to have a shot at it. Sometimes we get lucky. Sometimes we don't.

I assume you're asking for the original press release from Ontrack Data Recovery. And, helpfully, not linked from either the Slashdot summary or the Computerworld article.

I forgot 2006!

Also, here is Ontrack's official 2007 list. :)

here

Here is the list at the originator's site: http://www.ontrackdatarecovery.com/data-disasters-2007/?news=120407