Kevin Marks mentioned on the cc-metadata list that you can query Technorati for a list of recently syndicated content, grouped by Creative Commons license.
You can also drill down and get a list of recently syndicated content under a specific Creative Commons license.
The results are a bit rough now, but one can easily imagine combining license searches with keyword and other search filters to effect an ongoing search for specific licensed content. For example, tell me whenever a Creative Commons licensed image of horses is syndicated.
Marks, who works at Technorati, indicates this sort of capability is “an important future direction.” Other blog and search outfits take note.