After the release of Tapioca for Nokia 770, there have been a lot of discussion, specially on #maemo, about the status of SIP on the UI and the interoperability with another clients. There is a page at Tapioca Wiki explaining the current state of this supoort in the framework. A small quotation:
The main issue that has yet to be solved for full interoperability with other SIP clients is Media traversal trough NAT environments. That means that you can connect (establish a session) to other clients, but you will not be able to make Voice Calls to anyone.
I am currently working on adding support to other kinds of accounts, including SIP, besides Google Talk ones. I hope to have it working along with SIP support on Tapioca.