This new functionality is really exciting but it was only supposed to be rolling out to US customers:
We’re rolling out this feature to U.S. based Gmail users over the next few days, so you’ll be ready to get started once “Call Phones” shows up in your chat list (you will need to install the voice and video plug-in if you haven’t already). If you’re using Google Apps for your school or business, then you won’t see it quite yet. We’re working on making this available more broadly – so stay tuned!
Imagine my surprise when I saw this in my Gmail chat window this morning:
I don’t know if this will stick around in my account but I am keen to try it out! It is apparently pretty good and voice quality is great. I have to make a call to the United States later this afternoon and I think I’ll try this out instead of Skype.
Update: It looks like you need to have a Google Voice account to add credit. This pretty much limits the service for me because I haven’t been able to create a Google Voice account despite receiving an invitation a while ago.