Nice meditation on the role photo sites and tagging might have on the future of image search.
One of the powerful aspects of owning a top photo sharing site is the possibility to own image search in the future. Microsoft, Google, Ask Jeeves and Yahoo! all have extremely mediocre image search.
Search for a photo of the Golden Gate Bridge on Google image search and see what you get — super mediocre images of a great photographic subject. By owning an online photo sharing site Microsoft could use user online ranking data to prioritize image search and make MSN image search the clear leader. Although image search is a much smaller subset than text search it would be a powerful toehold in the Google dominated world.
Although I have not seen this type of thing appear at Yahoo! yet (and with the strong grass roots community spirit at Flickr you’d probably need user buy in and even a possible opt in approach) I suspect you may see it in the future. Yahoo! has a huge leg up owning Flickr.