A Google Earth KMZ file containing polygons in a latitude/longitude grid and labelled with the number of Flickr photographs they contain.
09/09/2006: I added clickable icons to the labels layer; you can see the bounding box used to calculate the number of pictures as well as a clickable link to view that area on a Flickr map page.
Update, 09/09/2006: The problems I noted with the data were partly my fault (0-based index vs. 1-based index error) and partly Flickr's (though they have since greatly improved the results returned by the web API.
Update, 09/09/2006: I noticed that the web API still isn't returning all of the data; check North and South Korea for an example of this.
I'm not quite sure about the accuracy of the data Flickr's webservice is returning; if I'm to believe the results it returned there are only 10,197 photos that have been tagged with an accuracy of 8 or greater (out of ~2.5 million at the time I collected my data). Accuracy level 8 is between "Region" and "City" according to the flickr.photos.search documentation. There also seems to be a couple of odd horizontal stripes where images show up that don't correspond to areas that you'd think would have high photo activity. In addition, I noticed while searching for photos in Seattle that they did not appear in the bounding box containing Seattle but rather the bounding box directly to the north of it.
You'll need Google Earth to view these files.