Just to clarify for those googling this, if you install Xcode you WILL get a fully functional option in the Spotlight preference pane (in 10.7.2 with Xcode 4). You can always delete the /Developer folder if you don't need the developer tools. Of course, since there's some type of official support for the option there must be a way to enable this without installing Xcode—or at least without installing all of it. I surmise it's either a setting in a defaults file or a modification to the search.bundle CoreService itself, although I wasn't able to figure out how to enable it without Xcode—not that I spent that long trying, since I need Xcode anyways. I'd be curious if anyone finds a non-Xcode solution!

Also of note, the aforementioned preference plist hack works however it will only save the setting as long as you do not open the Spotlight preferences.