See official gmail blog at http://gmailblog.blogspot.com/
for more details.
Gmail lets you call phones in US and Canada for free already.
It has just been announced that this deal has been extended for the whole of 2011 - a whole year more (originally only till end of 2010).
Hope people find this useful. I have used it on occasion and found the call quality quite good!http://gmailblog.blogspot.com/