There are already keyloggers for i.e. ubuntu (you have them in the official repository even), not sure if they are even in a scripting language (perl/python) or if you will have to recompile them to make it work in the n900.

But the main problem with the idea is that you don't need a keylogger, you need a fingerlogger, as good part of the interaction with the device is thru the touchscreen (even the on screen keyboard could play a part therel.