Apparently it happened last week, but few people noticed it till just recently.
Could be because not many people knew that those seven notes affixed to the construction scaffolding are the first few notes of "Never Gonna Give You Up." Or maybe it's because Rickrolling is soooo 18 months ago. C'mon, nerds! Get with the times! Next time try something more along these lines.
(For a much more in-depth look at the long history of MIT pranks, T.F. Peterson's Nightwork: A History of Hacks and Pranks at MIT is well worth a read, by the way.)