first of all. thanks that was one of the thinks i was missing on my N900.
anyway you said that you want to let your app seamlessly integrate into the N900 environment, that is a good idea. applying the FB colour scheme to your app would also be a good idea... but what ever you do, please do not choose other colours.
your idea not to implement the FB chat is something that i would clearly support. but maybe you could implement a link to the right apps on the N900 of each user. Pidgin and the normal chat app would be two programs to start with. maybe it could be user selectable in the options tab of your app, which of those apps should be started.

and my last point for now... if i could choose something that would be the most important function, it would be the "comment on other users status" and naturally also on the comments they did on your own status.