I am trying to edit a photo in Photoshop CS5 and get the error scratch disk is full. I have the scratch disk set to the Mac's HD and it has 60G of free space.
I am using CS3 still, but perhaps my experience can help.
1) Double check that you are in fact using the scratch disk you think you are. Go into PS preferences, and look there. For some reason last year my Scratch Disk preference was lost and it reverted to the boot HD. If it seems to be correctly set, un-check the preference and restart PS. Then go back and re-check your scratch disk preference and restart PS. Perhaps it just needs to be 'jiggled' back into place. I know, computer voodoo.
2) Repair permissions. Whenever I am getting weird errors with PS (which thankfully is not often) I find that in 80% of the cases repairing permissions on the boot disk (where my applications are installed - perhaps you have a different set up) seems to make the error go away. More computer voodoo....