Yesterday I was "tipped off" about a possible Flash-mob outside of Powell's books - it was hinted that they would freeze like some folks did in the Grand Central Station in NYC.
So at 3:15 I was there with my camera and they did it! It was weird, suddenly it got crowded, then they stopped, then just as suddenly they all started moving again. There were people eating bananas, kissing, pointing, taking photos, busking, smoking, all frozen.
I love Portland.
(Some more Flickr pics here, obviously the pictures don't do it justice!).
See also Oregon Live.