Spotlight Search
Snow Leopard incorporates a sophisticated search accessible through the magnifier icon at the upper right of the screen, the finder window, and keyboard shortcuts. It is capable of Boolean and date range searches. The search accesses all Mac-formatted and FAT32 drives. It does not access Bootcamp or Windows-formatted external drives. See Mac Help for more information on how to use Spotlight.
Embedded Metadata
Spotlight Comments
You can enter searchable information into files of all types in the "Spotlight Comments" field of the information panel. The field does not show up in other programs. To access the information panel, select a file, then select the i or "More info..." on the Finder window.
IPTC and EXIF
The information panel displays basic EXIF data and several IPTC fields. Data cannot be entered into the panel, however. All IPTC fields are searchable, whether or not they display. EXIF fields can be searched as long as you write them the same way they appear on the information panel, which might be different than they appear in Photoshop or other programs. (I have not tested EXIF fields that don't display in the Mac information panel--Steve.)
XMP
Snow Leopard's Spotlight searches all XMP metadata embedded in image files, including metadata entered in custom panels. There is no way to enter XMP fields directly in Snow Leopard, however. Other programs must be used for data entry.