Portal 2's co-op mode and, to ameliorate that experience, Valve has upgraded the quality of Steam's voice technology. In place of that busted old junk is the SILK audio codec, the same one developed by (and used in) Skype -- those cats should know a little something about internet-based voice tech. While the older Steam Voice required a steady 15 kbps of bandwidth, SILK dynamically adjusts its bandwidth demands, from 8 to 30 kbps.
During a brief test, we found the audio quality to be a notable improvement from the likes of Xbox Live, PSN, and even Steam Voice, but not quite "Skype-quality." Today's Steam client update brings the new codec, along with some other improvements, exactly one month before Portal 2 will require you to use it. We're hoping Steam users test it out before then, so Valve can iron out any pulls in its SILK stockings before Portal 2 day.