It was absolutely a foul, but Thomas Muller set a pick in the box that Jimmy B would've been proud of to free up Robben to slot home a beauty from a sharp angle. Barca got away with two hand balls in the penalty area so all was pretty even. Bayern thoroughly deserved to win 4-0, if not by more. (I know I'm biased, but that was pretty much the way most of the commentators felt on Fox Soccer as well.)
Here's the pick play. I wish it showed the finish, because it was a beauty that Robben tucked into the far corner.
I might feel worse about the pick if Jordi Alba didn't respond 5 minutes later by pulling this stunt.