Quite a few people have been asking for a generic VoIP client that can be used with any VoIP provider without using cell minutes (pure WiFi based SIP calls).
Here is a client that does pretty much exactly that. A BlackBerry (true & pure) VoIP client form fgmicrotec called fgVoIP. It can be used with any SIP capable VoIP provider including your own Asterisk/PBX both over WiFi and 3G data.
Currently supported devices are:
BlackBerry 8520 OS 4.6 & 5.0
BlackBerry Bold 9000 OS 4.6 & 5.0
BlackBerry Bold 9700 OS 5.0
BlackBerry Curve 8900 OS 4.6