Well, programs written in ARM assembly require an emulator to run on a PC, not that I recommend it to anyone as the first language to learn...

Python is complete, very portable and easy to learn. It has pretty decent performance for most things and there are ways to speed up parts that need to be fast (Psyco, Pyrex/Cython to name a few). The tablets support Python very well, but you need to install it separately (or install a program that uses it, like Canola).

Graphical user interfaces are one of the less portable aspects of Python. If you want to run your software on the tablet and on PC, you'll probably need to implement separate UIs for both. The tablet specific UI (Hildon) is not available on the PC, and the standard Python UI called Tkinter is not included with the tablet version of Python. On the other hand, a custom full screen UI can be implemented with a module called PyGame, which is available both on the PC (separate install) and on the tablet (comes with the rest of the Python).

There's a free online book called Dive Into Python. Check it out and maybe you'll like what you see.