Leopard breaks the ⌥⌘0 keyboard shortcut to View > Actual Pixels in Photoshop.

Open the "Keyboard & Mouse" preference pane and go to the "Keyboard Shortcuts" tab. Uncheck the box next to "Zoom". It should now work again.

Leopard overrides the keyboard shortcut (actually ⌥⌘=, but that seems close enough, at least with my Swedish keyboard layout…) for zooming in. Even if you turn this keyboard shortcut off, you can still zoom by scrolling while pressing ⌃, if this option is activated under the "Trackpad" tab. Not sure how things work on a non-notebook.