RIM has an application called Mobile Voice System
which is their Fixed Mobile Convergence solution which currently works with Cisco Unified Communications Manager and it says other "IP PBXs" but nothing else specifically. It looks extremely cool however, there is no OCS support. The only OCS support that exists as of 4.1.6 or 5.0 is their CWA R1 interaction for instant messaging and presence integration. So they aren't even up to date only their OCS IM.
I talked to a person at RIM as recently as August 10th because apparently there was some sort of MVS road map presentation in July but there still is no support.
Again one of the many examples of RIM taking Microsoft integration for granted, no OCS R2 IM, no OCS MVS, no Exchange 2010, and fairly poor exchange integration (compared to winmo activesync).
Good luck. And if you do some how get it to work via a Linux PBX let me know.