When you first use Adobe Photoshop CS5, you get asked what you want to do about the command-H (⌘ H) key combination.
If you selected 'hide photoshop' but you regret it and want to use the traditional Hide Selection instead, you need to change your keyboard shortcuts.
In Photoshop select edit > keyboard shortcuts
and then scroll down till you find "Hide Photoshop".
Click onto the shortcut, and then click 'delete shortcut'.
Next scroll further down to View > Show and in there you find 'Selection Edges'.
Click in and type "cmd H" and you'll see ⌘-H. Click OK and you're done.