"As rumored last night, Google Voice is now embedded within Gmail, allowing users to make and receive calls to their Google voice numbers, from right within their browser. That's pretty impressive."
I tested this on Wednesday afternoon, and it worked great. I thought it was going to depend on either flash or HTML5, but in fact, it uses its own plugin, and it should run in any browser. I tested it in Google Chrome, Safari and Camino, and it worked fine in all three. You can go to http://www.google.com/chat/voice/ to download the plugin. If you want to receive calls in Gmail, you'll have to set it up as a destination in Google Voice.