I don't use Photoshop that often and it had been working fine until today. It seems that I can Save and Save As..., but if I try to Save For Web I get a write permissions error. Also, upon closing the program, I get a prompt that my Preferences can't be changed dur to the incorrect file permissions.
So something has changed with the permissions on your system.
Start by checking your preferences folder in ~/Library/Preferences/...
What should I be checking? Are there particular files I need to look at? And what should they be set to?
Check that you have permission to read, write, and create files in that folder and the Photoshop prefs folder.