If you are using Microsoft Entourage (Outlook for the Mac) take a look at the latest update. Quite a few upgrades but most importantly "with the new 11.2.3 Update installed, you can now use Apple’s Spotlight feature to execute searches against your Entourage database contents. Spotlight can find any item inside your Entourage database; it does not matter, where that item is."
Interestingly, I didn't find out about this from any official source - rather from bouncing around blogs after two weeks of abstinence.