Google Turns Gmail Into a Phone
It would seem to be a natural extension of the voice chat functionality, and Google Voice as well. While you can use Google Voice to make a call, it doesn't use VOIP to do so: it calls your phone and then calls the recipient as well.
Obviously this sort of functionality has a high-profile target, Skype (or maybe two, Fring). Launched from the chat section of Gmail, it shares a lot in common with the Google Voice UI.
Google says that calls to the U.S. and Canada will be free "at least through the end of the year," with other calls billed at their "very low rates." If you have a Google Voice phone number, calls made from Gmail will display that number as the outbound caller ID. You can even receive calls to that number inside Gmail.
Still confused? Check out this Google demo video: