Symptoms: Every time you start Photoshop, you receive the prompt to confirm the deletion of Photoshop's preferences, or
Photoshop tools act strangely, as if a modifier key was held down all the time: you can only draw straight lines, you sample colors rather than paint/fill, the cursor has a + sign, etc.
AND: You are running Mac Os 10.6.5 or 10.6.6.
Solution: Update to Mac Os 10.6.7: As explained in http://kb2.adobe.com/cps/881/cpsid_88159.html
Apple had made a change in 10.6.5 in one of its APIs that affects other programs (Photoshop does not use it), and the result is that they interfere with Photoshop, that receives information that modifier keys (CTRL, Command, Option, Shift) are pressed, when they are not. Adobe reported the bug to Apple, but it was too late to be included in 10.6.6.
The solutions are
a) Upgrade to 10.6.7
b) An Adobe Employee has made an unofficial interim fix for this bug. You can download it and have more information on this post: 72.
This is one of those cases where Photoshop has the symptom of a bug, but the cause of that bug is not in its code.
Note that the post to reset the preferences or reset the tools to their defaults when they are acting erratically is located here: