Something like this should have come earlier, many say. But as I write, it's not even under beta. Techcrunch managed to get a leaked Mac version of the software, which seems pretty ready for release, although quite basic. The release will probably be for soon, since Google opened the GV service for everyone to register without having to be invited to do so.
In fact, Google was reportedly going to sacrifice the desktop client for a web-based client, because Google is focusing more on browsers. But they changed their mind after many users petitioned for a desktop client. A softphone is so important, especially for Google Voice, since it allows people to use the GV number more flexibly. You could, for example, receive calls free through your Google Voice number over your computer, and this would be handy while travelling abroad. Watch Techcrunch's video of the leaked GV app.
Another data point for the increasing relevancy of VoIP for the consumer market. The change is upon us.