I think its a bug. I recently formatted my Mac and did a clean install of Tiger and iPhoto 06, yet none of my library is indexed by spotlight.
iPhoto 5.0.2 supported this feature - the thing was that the photos needed to be tagged with keywords after that version (5.0.2), so your older photos would not be indexed. The workaround was to create a new keyword, tag all your photos with that keyword, and then remove that keyword and all your photos would be indexed.
I am not sure if this method works in 6.0 but you can try doing that. I'll try that tonight and see if it works.