It wasn't even a month ago that Steam received an update that improved voice chat quality. Now, today, Steam once again has received another major update this time improving voice chat even further. The update adds in a codec (a method in which audio is compressed) that comes from popular online voice chat software Skype. It's called SILK and not only should voicechat use less bandwidth, it should sound a lot cleaner and be more reliable.
This update affects the Steam client itself for starting voice chats, but it also affects the Steamworks API. This means titles like Valve's upcoming Portal 2 will utilize this improved codec for better co-op communication.
The Steam update also includes a handful of bug fixes as well, you should be prompted to download the update next time you start Steam.