I haven't found this anywhere on the forum so here goes:
Here are instructions on how to break spotlight in one easy step:
Open iPhoto, find a picture you know you have edited in the past. Click "Revert to Original" and vóila!! Spotlight will now be completely unable to find any keyworded images!!
Nice one Apple.
There is a fix on the Apple support forums & loads of people seem to be having various Spotlight related problems.
The fix should you need it is:
1. Open iPhoto and create a new Keyword. "Spotlight" is ideal.
2. Select all of the images in your library then assign them the new keyword.
3. Quit iPhoto. Spotlight now re-indexes all the images in your library and works again for all the keywords you may have.
4. Open iPhoto and go to Keywords in preferences.
5. Delete the "Spotlight" keyword you just created and this removes it from all the images in your library but Spotlight still works for the rest of your keywords & the problem is fixed.
Considering Spotlight is marketed as the killer app, you'd have thought this fairly easy-to-duplicate bug would have been found straight away...