I'm not really familiar with twitter, but from what I read, Twitter to statusnet (=identi.ca) needs 3rd party help at the moment - like posting from a client that supports it or posting to a web service (like ping.fm) that will send the tweets to both.

The other way (posting to statusnet account, sync to twitter) works out of the box, but isn't really an option if twitter is your main channel. (you'd miss all the buzz your tweets create ...)

the developer roadmap for the statusnet software says "pushing from twitter" is in the works (so sync would work in both ways without 3rd party support), but i haven't found information on when they think they'll provide it.