This rumor does not have any sort of backing at the moment. But it does make for a logical extension of the current Google Voice service. According to reports on the GoogleSystem blog, the company is in the process of testing a new voice call feature on Gmail that is likely to make the current chat feature even more useful.
As you know, currently Google USA users have to either use a phone or the Google Voice website to make voice calls. The new Gmail voice call feature could just be too useful. Of course, it is not clear how Google would integrate the feature with a service that is being used the world over (since Voice is USA-only)
Nevertheless, this is how the feature supposedly look like. Take it with a pinch of salt.
[via Google System]