Today I found a new VoIP/SIP application for WM5/6! It's called PortSIP (only been around for ~3months)
General info: http://oldsap.blogspot.com/2009/04/portsip.html
Official website: http://www.portsip.com/
Support/Suggestion/Advice forum (which the developer(s) actively read/respond to): http://forum.xda-developers.com/showthread.php?t=508715
I've used it with 2talk and the quality is fantastic.
What I really like is that PortSIP connects directly to 2talk (or what ever sip provider), portSIP doesn't connect to a central server in the USA/UK like fring does (which adds ~400ms!).
The UI is really basic, but it does the job. The developer(s) are also constantly adding new features.
I've only used it on my home wifi connection for now, I'll try it on 3g (vodafone) this afternoon.
Hopefully some of you will find it useful

#
