A new Google Apps Training presentation includes this intriguing screen shot of the Google Talk application that strongly suggests that calls to regular phones and / or SIP support might soon be available in the Windows client.
Google Talk has supported gTalk-to-gTalk VOIP calls for a while, but not calls outside the Google Talk network. The new interface suggests you'll be able to type a number in to the client, or use a 'dialpad' to get a more familiar phone pad interface.
Obviously these new features are still in internal testing, but the use of this screen grab in a Google presentation suggests it might not be too far away.
If Google do start offering calls to the standard phone network or SIP clients via Google Talk the big question will be if they offer it as a paid, free, or audio add-supported service.