[Brian]

Snow Leopard makes some changes to the way that service shortcuts work compared to previous versions of Mac OS X.

To re-enable your keyboard shortcut, do the following:

1. Open System Preferences from the Apple menu.
2. Select the "Keyboard" pane, and switch to the "Keyboard Shortcuts" tab.
3. Select "Services" in the list on the left. (Next to the icon that looks like a gear.)
4. With "Services" still selected, scroll the right hand pane until you see the "Text" grouping.
   (There's a bug in 10.6.x which can cause this pane to be blank; see this post for what to do.)
5. Continue scrolling in the Text group until you see the "Define in OmniDictionary" entry.
6. If the box next to that item isn't checked, check it.