Just a quick note. I was wondering about this as well and was having very little luck cross compiling gcc. However, I came across the following post:

HOWTO: C/C++ compiling on the unit itself.

And have managed to get it working on the n810. This doesn't have any note about java, however.

The main changes are to load the files from

http://repository.maemo.org/pool/chinook/free/

Rather than ../pool/maemo2.2/...

I have managed to compile a simple hello.c. Probably try something a bit more complicated later on.