Despite the game only being out for a week and a half, Valve has already released the second patch for the PC version of Portal 2.
The patch was released to fix issues with non-joystick hardware registering as a joystick, to help improve handling of Mac OSX crashes and to fix crashes and/or graphical glitches when changing graphics setting without restarting. You can find a full list of the patch in the source provided.
Valve is also coming out with a free DLC pack for Portal 2, which contains more test chambers for single player and co-op and a challenge mode just to name a couple. It will be released sometime this summer.