The new service is available via a plug-in, currently compatible with Mac OS X, Windows XP and Windows Vista. While it won't (yet) support Linux users, they are offering a variety of browser support, including Firefox 2 and 3, Safari 3, IE7 and of course Google Chrome. Opera isn't mentioned so it may have been left out at first.
This is certainly a very interesting endeavor and it's easy to speculate on where it could go from here. Perhaps smartphones and PDAs of the near future could be used to video chat via Gmail as well, or maybe Google has the Android platform in mind as a target for a Gmail communication device.