"For many Photoshop users, there is a horrible bug in Adobe Photoshop that causes the program to stop importing from the clipboard. You’ll be using the software for hours or even days without a problem, and all of a sudden you won’t be able to copy and paste anything into it at all, with your only option to close and re-open Photoshop."
http://microsoft.blognewschannel.com/archives/2007/09/05/how-to-fix-the-photoshop-clipboard/