Occasionally when I switch (via cmd+tab or Expose) from photoshop to another app then back - all of the Photoshop menu items are grayed (inactive state). You can still issue hotkey commands (e.g. cmd+O, cmd+w, etc).
This doesn't happen every time I switch apps, but I haven't noticed a specific action that causes it to happen consistently.
OSX 10.6.7 Photoshop CS5 12.0 x64 Wacom Intuos 2 (I only mention this, because it seems like it only happens when using wacom tools mouse/pen)