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:
- Open System Preferences from the Apple menu.
- Select the "Keyboard" pane, and switch to the "Keyboard Shortcuts" tab.
- Select "Services" in the list on the left. (Next to the icon that looks like a gear.)
- 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.)
- Continue scrolling in the Text group until you see the "Define in OmniDictionary" entry.
- If the box next to that item isn't checked, check it.