iPhoto and Spotlight
In Mac OS X 10.4 and later, you can use the new Spotlight search feature to search your computer for various files, including the photos you have stored in your iPhoto libraries. This is made possible by iPhoto telling Spotlight to reindex the iPhoto library whenever changes are made to some of your photos. This way, Spotlight can keep its search information up to date, and you can find your photos by searching for their titles, keywords, or any other info associated with your photos in iPhoto.
However, it does occur sometimes that Spotlight will not update its index of one or more of your iPhoto libraries, causing your Spotlight searches not to be able to find your photos successfully. The most common time this happens is when you copy a library to or from another disk or computer, but it can also happen occasionally just in the normal day to day use of iPhoto.
If you find that Spotlight is not properly finding photos in one of your libraries, you can select that library in the main iPhoto Library Manager window and select "Force Spotlight Indexing..." from the File menu. This will tell Spotlight to reindex the whole library so that it can provide accurate results when searching. The reindexing may take a minute or two to complete, after which you should see your photos appear when searching for them.