Spotlight should be able to find pictures in your iPhoto library, if you are using iPhoto 5.0.2 (or later, I presume). However, if you kept iLife up to date and upgraded to iPhoto 5.0.2 *before* installing Tiger, you'll find that Spotlight never finds any of your pictures. The fix is simple.
- Quit iPhoto
- Delete ~/Library/Pictures/iPhoto Library/iPhoto.ipspot
- Start iPhoto
- Use Spotlight to search for keywords or comments on your pictures.
The iPhoto.ipspot file just contains the path to your AlbumData.xml file and gets recreated (presumably after iPhoto has updated something else).
[robg adds: My iPhoto was upgraded prior to Tiger installation, but everything seems to be working fine. I have heard reports of no iPhoto indexing, though, so maybe this will help someone...]

