Yes. The N900 is the best phone for onboard development by orders of magnitude. No other phone comes close.

On native Maemo you have Python, shell (bash), full java JDK, you have C and C++ compilers, etc. And I'm sure there are many more.

But then, you can install Easy Debian, and when you do that you will be able to program in any language you can program in on a desktop Debian environment. PLT-Scheme, Lua, etc. You can install multiple versions of Python. You can install Ruby.

As a matter of fact, you can install gvim or emacs, and use them as programming environments too.

Yeah, the N900 is a programmer's dream come true.