Get yourself a copy of 'Documents to Go' or the competing products 'iSilo' or 'Quickword', and then a keyboard to fit your PDA.
I have a wireless keyboard with my Palm zire 31. I went for the 'official keyboard' but even the old cheap 'can get it on ebay for $5' wireless that I used with my Palm V works with the newer palm (it just doesn't have a form fitting cradle that fits the new form).
With that, I did all the in-class note taking I wanted during my undergrad degree, and kept it for notes during game sessions until I finally started using my laptop for the internet access...
There are a lot of DnD themed apps out there for PDAs, though I personally feel a PDA is a better solution not for the people who like lots of apps, but for those of us who want to able to have and make quick text notes as the game moves along.
I write my plots as they happen, with a lot of it based on what is being said by the others around me and how that hits my internal thoughts... so something to jot down quick notes is very handy.
I have a small laptop (iBook 12"), but the PDA is even more portable and I tend to pace the room when I GM, making the PDA the more convenient choice save for when I'm running something that I want online acces for, like getting to my kalamar name generator back when I used to run that or getting to my 'Cyberpunk genres lifepath generator' when I run science fiction, such as my
Mosquito setting.
Actually using apps on it is something I could do, and I did download a freeware dice roller once, but I tend to prefer gaming without such apps and the dice roller goes unused (so its a good thing it was freeware...

if I knew its name off hand I'd type it in, but it and other apps are extremely easy to find on Palm app download sites. It was a pretty good roller, but I prefer the plastic thingies).
Note: Last time I checked about a year ago, there were stores selling -new- Palm Vs for $20-30 on eBay. they're probably still at it. Anyone wanting to try out this kind of setup could get a very cheap old generation Palm and wireless keyboard and see what they think. If you find you really like it as a portable word processor, which I did, you can then budget in a newer model or just use your cheapy until it breaks and then get another...
I rarely use my palm Zire beyond the word procesor and the calendar.