Google confirms the acquisition of Gizmo5, a proprietary SIP soft phone. "The Gizmo5 network uses open standards for call management, Session Initiation Protocol (SIP) and Extensible Messaging and Presence Protocol (XMPP). However, the Gizmo5 client application is proprietary software and uses several proprietary codecs, including GIPS, iSAC, the wideband adaptive codec made popular by Skype," explains Wikipedia. Gizmo5 is "the only SIP service without PSTN-based US phone numbers that may be used with Google Voice."
"While we don't have any specific features to announce right now, Gizmo5's engineers will be joining the Google Voice team to continue improving the Google Voice and Gizmo5 experience. Current Gizmo5 users will still be able to use the service, though we will be suspending new signups for the time being, and existing users will no longer be able to sign up for a call-in number," mentions Google.
I wouldn't be surprised to see Gizmo5 morphing into a new version of Google Talk, with built-in support for Google Voice.