Switzerland-based O-FONE (not to be confused with Microsoft’s oPhone) has released a SIP VoIP client for Symbian’s S60 platform.
Perhaps more interesting than the client itself is their claim that they’re the first to bring SIP support to S60 devices. From this page:
“Download the first Symbian mobile SIP client in world” [sic]
Unless they mean the first Symbian mobile SIP client purposed for O-FONE, I think they might be forgetting about Fring. Fring, which supports SIP (along with Skype, Google Talk, Twitter, AIM, and Yahoo), has had a Symbian client since March of 2007.