PDAs as a College Notebook?

Eugene asks: "G'day everyone! Here's the deal, I study Engineering in college and therefore, I have to write down LOTS of mathematical formulae and such. Now I heard that students of Law/English/etc. find great use of various PDAs as a notebook replacement(that's pen&paper notebook). I'd like to know if there's a PDA software-solution for quickly writing down math expressions( Something like the equation editor shipped with MS-Word - but if possible with a more intuitive way of entering data). All I could find so far are lots of calculators, that do little in the way of easily entering equations and storing them for later review." Well, that would be one less thing to lug around in the ole backpack. Now if we could only get textboox in digital form...