Actually (trying to get in this discussion way late, having found interesting arguments in both sides), first (as always here) it is good to define exactly what is choking, as Federberg put (and others too, I guess).
From the outside, it is impossible to distinguish bad execution from choking, as choking is merely an explanation for bad execution. What we mean by bad execution here is "normal" bad execution.
Of course, in general one chokes in important moments, big points -- but as it is a psychological thing, sometimes you choke in apparently absurd moments -- precisely the moment were you understand you are in an important/winning position, which could be something like serving 30-15 at 4x4 in third set, and realizing that you are playing better and you are supposed to win.
All this to say: it is hard to tell. My point before was that del Potro made it quite hard for Federer, and the lost game at 5x4 is understandable, specially taking into account the precise Haelfix's arguments. But, as GSM put, Federer serving stats on the tie-breaker were atrocious, his whole display was terrible, while he was the better player so far in the third set. The drop in form is there.
But he could be angry that he let it slip on that previous game... it could be a lot of things. From the outside, is hard to tell. Only the guy that felt (or not) that burning stomach can tell. All in all, I personally classify it as "relative choke", but just in the TB. I'll give a pass for the drop shot (even if the drop shot is the classic move by chokers who gave up fighting on the point, on the other hand it was part of Federer strategy).
What I think is interesting to analyze as well (
@El Dude, jump in!) is, if we admit Federer choked, how the hell one guy that won 20 majors, just won the last, just got to #1, chokes to close out a (relatively speaking) smaller tournament that he won tons of times? It would tell a lot about the player -- that those guys are completely and utterly addicted to winning, for example.