Why does mdworker keep spamming the logs on Lion?
The messages are coming from Spotlight search.

The message themselves are in relation to iPhoto's data on your images.  The tags can be found in the AlbumData.xml written out by iPhoto for sharing use (such as Spotlight and other media sharing tools)

Without more information about the version of iPhoto in use, or whether you have any issues with iPhoto itself (and Spotlight's integration) there's not more more I can tell you.