Why make a handheld PC? You still need power.
i figure I'll just run off a Li-Ion battery (after finding or making a circuit board that gets it to the appropriate power output and type)
Li-Ion batteries are very complex. Don't make your own unless you know how to make some kind of power tracker/regulatory circuitry.
hmm..thats true...LiIon+bad power control = fire occasionally....
i figure somebody's done it before though, and if they did, they've probably written something about it on the web somewhere.
I've looked into it recently, and its true...a Mac mini is about the smallest machine you can buy. Sometimes they have good deals on refurbed ones..
You'll need 19 V IIRC to get the thing to boot, but there are some relatively cheap Li batteries available, and you might be able to find some plans on how to hook them up to the mac mini.
Theres plenty of resources for mac mini mods (giyf), and if you really wanted to go ultra portable, you could always opt for a CF-to-IDE, and get a large capacity CF card and use that in place of the hard drive.
A ground up nanoITX solution will be longer and wider than the mac mini, but its possible you could make it just as thin.
Let me know how this works out for you as I an interested in a similar project...
after looking into it a little more, I remembered I could put windows XP on it by bootcamp. Then I could attach a battery, small LCD, and maybe fix a small keyboard and touchpad (i figure the keyboard an touchpad could go under the screen like on a laptop and the battery could go on the bottom or something).
So, what's your project?
EDIT: nanoITX is a little less than 5"x5", while the mac mini is 6.5"x6.5", so it might be able to be smaller, but you're right, it would be hard to fit everything in and still be smaller.