Hi
I also got my N900 a month ago and been reading this site for some months.

Does this Quake3 version support mods? When I try to load one, the whole app just closes. I really want to try one mod. Secondly, can the author implement aiming with the accelerometer? I used to play Quake 1,2,3 with accelerometer on my N82.... It should work as a mouse, it makes the game much playable.

Have you enabled extras-devel? it's there.
It's not good to install much stuff from there...
Here's the repository details. First go to Application manager > Application catalogs > New

web address: http://repository.maemo.org/extras-devel/
distribution: fremantle
components: free non-free

Then you'll see ioquake3 in the refreshed list. Hope it helps