Alas, it was a Pennsylvania Army National Guard pilot who did that landing. He works for Keystone Helicopters as an EMS pilot on his regular employment I believe. The task was to retrieve some captured enemy combatants and that was the best location he could find. The troops had another site in mind but the Chinook pilot elected to use this one.
Either Helicopter Pilot International in DC or his employer had a report about the photo and the pilot involved.
Technique would not be all that hard for a Chinook. Lock the Parking Brake, Land the rear gear as in a normal hover landing....adjust two inches aft cyclic....maintain heading with pedals....maintain pitch attitude with the Thrust Lever (collective). Finding something to look at would be a trick...but doing it using instruments primarily can be done.
As people walk to and fro...the CG changes slightly (allowable range of CG travel is about 144 inches)as weight goes forward, more power is required...and as weight shifts aft...less power is needed....all done with very small movements of the Thrust Lever.
It would be one heck of a time to have an engine failure however.
Could not find the write-up I mentioned...but this non-commercial web site for Chinook enthusiasts is great.
http://www.chinook-helicopter.com
This link has a copy of the photograph and a brief discussion of the circumstances surrounding the event.
http://www.snopes.com/photos/military/rooftop.asp
Different angle in photo and much better write-up about the event.
http://www.defendamerica.mil/article.../a111703f.html
Hope that helps you out BRL.