Saw Now You See It (the movie with Ruffalo and Morgan Freeman) and I think he could pull it off.
He was good in that and he already has a rumpled look. as long as he puts his spin on the character and doesn't try a full on impression, I think he could do it.
Ted, back me up on this buddy! I've been saying for years (literally) that Ruffalo should play Columbo. He's got the look, the brains and I think the integrity to help deliver a truly great Columbo.