After two months of beta testing and several updates, Valve has announced that the so-called "UI Update" for their Steam client will finally roll out to all users on Monday. Far from being just a visual overhaul, the update ditches Steam's Internet Explorer-based core for a new WebKit-based rending engine, and will bring a few new features like syndicated gaming news feeds, a new in-game overlay with more context sensitive information, and customizable games libraries.
If you are just now finding out about the beta and don't want to wait until Monday, all you need to do is head over to File>Settings, click "change" under Beta participation, and then choose UI Update. The beta should download automatically and will be available after restarting the client.
Valve is also readying a Mac client of Steam, which will offer native Mac OS X versions of Left 4 Dead, L4D2, Team Fortress 2, Counter-Strike, Portal and the Half-Life series when it launches before the end of April. However, this is expected to come in separate update, so Apple users might still have to wait just a bit longer.