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You hope. It's mostly the largess of the controller manufacturers that let you get away with this. They could insist on strict RS232, and you'd be out of luck. Heck, 0V is explicitly in the center guardband of the RS232 signal.

But the real reason not to do this is because the whole point of the wide voltage swings in 232 is to help mitigate signal interference in the single-ended signaling that 232 uses. If you're running a longer signal cable in a noisy environment, you want every bit of protection the standard can muster.

And, if you must, there are several TTL to 232 voltage converters available, such as these
http://www.bb-elec.com/Products/Serial-Connectivity/Serial-Converters/TTL-Converters.aspx

One example was this, which just flat out says discontinued:

http://www.gefen.com/kvm/dprod...

Another is this, which mentions wireless but none of the models seem to support wireless:

http://www.bb-elec.com/Product...

For that last set, the manufacturer's page has no wireless ones either, except in the discontinued section:

http://www.icron.com/products/...

You need a PLC configured with the right I/O (relay in/out, analog in/out) with Ethernet and a common, open protocol like ModbusTCP... Automation Direct is a good supplier of these.
Alternatively one could use "dumber" remote I/O devices like these

Come to think of it USB ports that adequately present themselves as a CDC ACM device might just be a better idea.

You may want to check out B&B Electronics. A few years ago, they were dealing with custom boards, not sure if they still do now, but you can check em out at http://www.bb-elec.com/

Get real RS232 from a real RS232 company, B&B. They have been doing RS232 gizmos of every sort since the 80's. PCMCIA, USB, parallel to RS232, multiple ports, long distance extenders, RS422, RS485, serial for PDA's, you name it. Not cheap though.

Rather than have a separate PC server at every scan location, you could buy USB scanners like the Fujitsu ScanSnap or Xerox Documate and use USB over Ethernet hubs (example) to connect them as local hardware devices to a single PC server responsible for handling scan requests and routing the documents appropriately.

...rather than going into a long diatribe describing every design decision that led us to "ethernet hard drive", I figured it would be best to keep it simple.

I'm not sure RS-232 can make it to 1Mbps, though these guys sell interfaces that go half that fast. Perhaps you could shotgun? You should talk to them in any case -- they seem to know a lot about data collection.

RS-422 can go up to 2Mbps. I know this because I used to work at Convergent Technologies, which had a proprietary network based on this protocol. At about the same time, Mac people were beginning to use AppleTalk, also based on RS-422. Ethernet and Token Ring swept this all away, of course.

One last stab before I admit I'm out of my depth: instead of an ethernet-powered hard disk, get one of those shoebox computers. Pull out the power supply, and replace it with a storage battery that recharges off of solar cells. If you've got enough sunlight to power your collection device, you should have enough for the computer, assuming its got a low-drain CPU and one of those clever hard disks.

Connecting these two boxes 25 feet apart is the big problem. Wouldn't Ethernet draw more power than you could spare? But now I'm definitely out of my depth.

Try out the 232DRIO RS-232 Digital Relay I/O Module. It worked ok for me, easy to use. They have some other models as well.