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: