New in 10.5 is the ability to add a keyboard shortcut for the Show Package Contents contextual menu item in the Finder. Simply open the Keyboard & Mouse System Preferences panel, select the Keyboard Shortcuts tab, click the "+" button, choose Finder as the Application, and type Show Package Contents as the command name. Set whatever shortcut keys you want to use, such as Command-Control-S, then click Add. That's it; you've now created a contextual menu shortcut for Show Package Contents.

In 10.4, the only way to add a keyboard shortcut to Show Package Contents in the Finder was with a combination of apple script and automator as detailed in this hint.

[robg adds: With the ability to set these keys on the fly now, you don't even have to restart the Finder. I tried creating shortcuts for the entries in the More sub-menu, but sadly, that didn't work (so no keyboard shortcuts for Automator workflows via the Finder).]