Josh, don't get me wrong, I'm certainly hoping Photodune succeeds...
I'm a small fish so my opinions don't carry a lot of weight. But based on my experiences and observations over the last couple of years, I wouldn't go exclusive with any agency, because the future is just too cloudy. And I don't see much point in offering specific images as exclusives at an agency - unless maybe that agency's market was distinct in some way, such that the images would do exceptionally well there compared to other agencies.