I'm voting the same way as alberthrocks.

I hope it will come back later on. If it's still of any interest for you you should try to talk to Uber Spire for the Linux stuff.

I'm going to need some help. What buttons/functions should be on the keypad, and what functions should just be accessible through menus?

I like the clamshell design, but I think the 84+ shape is the best bet.
Still, what should I have accessible through 1 or 2 button presses, and what should I have accessible through menus?

I think I'll go with Voyager 200 shape (but no QWERTY keyboard).

And I got a reply from uberspire, he doesn't think he can help me much, but he did give me a nice list of things to know to make a Linux OS for a graphing calculator.

1) Get comfortable and learn how to use Linux. Have a strong understanding of how things are organized or how they work under Linux.
2) Learn C. If you don't have much experience with programming, learning C might seem difficult. There are lots of free tutorials if you Google "C programming tutorials". A good book ($20 used): http://www.amazon.com/Programming-Language-2nd-Brian-Kernighan/dp/0131103628/ref=sr_1_1?ie=UTF8&qid=1309227849&sr=8-1
3) After learning C, learn how to build a GCC toolchain. I'm assuming that you're trying to use a ARM processor, right? You'll need to build a GCC toolchain that'll compile for ARM targets.
4) Learn about ARM architecture, or at least have a strong basic understanding of microprocessors and how computers work internally.
5) Have an understanding of electronics and hardware. You said you want to modify the TinyCore to work with graphing calculator buttons. What you'll need to modify is the kernel. More specifically the drivers bundled with kernel. You might even have to write some drivers yourself, it'll depend on the hardware. A strong understanding of the hardware and electronics will be necessary to write the missing pieces of code to make the Linux kernel work on your hardware.

Maybe you can first try a hardware emulator (if you find one that is compatible with all of the microships you are going to use). There are some freeware emulators, but I think the best ones aren't free (only the legal versions of them, actually ), but they should cost less than buying all of the hardware anyway, so that might be the best choice. An extra avantage is of cource that an emulator can be reset, while when you break real hardware, you'll have to buy new.

A friend of mine knows a great emulator. I'll ask him if you want. (might take a while. I haven't seen him IRL for almost a year, so I'll have to send an email)

Ok, so I think I might need to work on the hardware, and just boot a system first, before making an OS...