B&H is selling a used FEB 05 2008
As for image quality, even wide open it's quite lovely. Stopped down to f/8 and f/11 it's actually quite remarkable. How remarkable? From midtown Manhattan we were able to read the street signs on the corner of JFK Boulevard East and 43rd St. in Weehawkin New Jersey when viewing image files at pixel resolution.
The lens weighs 36 pounds and there are probably less than 20 of them in existence. (thx, rob)
Update: Canon even made a 5000mm lens. (thx, sadat)
Update: Ok, last one and then we're on to telescopes. (I'm kidding...please do not send me links to telescopes.) The Panavision 300x HD Lens...2100mm. (thx, philip)