So. Some researchers at Cornell actually answered this, well sort of. They took 35 million photographs from Flickr and then analyzed the geo data as well as tags to see where people were taking pictures. They also made some awesome maps like this world heat map In the end the top locations according to their research(PDF) were: 1. Eiffel Tower
2. Trafalgar Square
3. Tate Modern
4. Big Ben
5. Notre Dame
6. London Eye
7. Empire State Building Which is in my opinion an oddly London-centric list.
As has been pointed out elsewhere, Flickr has a western bias. And where are the natural landmarks? My hometown chimes in at 23rd on their list, somehow. Emo's, which is apparently the second-most photographed location in Austin after the capital, is a dive music venue in the midst of Austin's "strip" that doesn't even technically exist anymore. Amazing place, but seemingly out of place on that list.
The third "location" for Austin is SXSW, which also probably explain's Emo's. I imagine flickr explodes with photos from SXSW. And yes this all backs up the argument that flickr is a flawed data source, but I think the study is still interesting.
And, this reminded me of another project on flickr where a user is using geotagging to show where locals take pictures compared to where tourists take pictures (he makes this distinction based on how long a user uploads photos from a given city). One of my rather biased favorite examples: Washington, DC