I ran into this same issue a while back. The solution to get print screen and the copy/paste clip board working again in Photoshop (CS (v8), CS2 (v9) and CS3 (v10)) is to run the registry fix below to force Photoshop to always check the clipboard:
Windows Registry Editor Version 5.00
[HKEY_CURRENT_USERSoftwareAdobePhotoshop10.0]
“AlwaysImportClipboard”=dword:00000001
Windows Registry Editor Version 5.00
[HKEY_CURRENT_USERSoftwareAdobePhotoshop9.0]
“AlwaysImportClipboard”=dword:00000001
Windows Registry Editor Version 5.00
[HKEY_CURRENT_USERSoftwareAdobePhotoshop8.0]
“AlwaysImportClipboard”=dword:00000001