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

You can download a 10 day trial version at their site:
BlackBerry VoIP - Mobile VoIP clients for BlackBerry

or see it in action on youtube:
YouTube - fgVoIP SIP VoIP client for BlackBerry with different VoIP providers

Hope this helps someone