Are you referring to the panels on the right? Unlike in CS5, in order to collapse a panel, you must double-click on the panel's tab (title), rather than beside it.
Edit: Nevermind the above. I see what you're asking now. It seems to be working fine for me. Have you tried clearing your preferences? http://forums.adobe.com/message/4209421#4209421
No, I'm talking about Panels you invoke when you click on a Panel Icon.
To the left of the right panels (Layers, etc.) there's another bay for more panels, these are usually reduced to mere icons so that don't take up a lot of room.
In Prefs, there a Auto-collapse Iconic Panels checkbox. When used this is supposed to make those panels collapse as soon as you click away from them.
That feature isn't working for me right now.
Seems to me there's an "Auto Collapse Iconic Panels" preference somewhere. See if you can find that. My Photoshop is in the middle of a 20 minute long deconvolution (via a plug-in) right now, so I can't check.