For me it has worked after I re-assigned the key in setup dialog. I chosen Enter again and now I can activate chat. Yes, with Enter key .You may want to try the same and see what being stored as key to config and maybe provide it by default to n900 users, etc.

There are dozen of ways:
1) Use camkeyd, it so convenient to invoke task manager.
2) Or hit Ctrl+BkSpace, also calls task manager.
3) You can switch off fullscreen mode
4) You can start camera and quit it, returning to task manager.
But yes, quit could be more convenient and obvious though.

I also set up 800x480 and it worked for me. I used 0.29-1 version from devel repositories, not that one from manasource site.

Idea: maybe it's good to adjust default config file for n900?

In general it works and I can chat, attack monsters, etc, FPS is not very great but quite playable. At least I can walk around Hurnscald, kill couple simple monsters, etc .