the NFL rule is similar to the rule in baseball that if a player catches the ball and falls, and the ball squirts out of his glove, then it is NOT a catch...
How many times have we heard this NFL rule stated as "the ground cannot cause a fumble"...
well that means if a runner has possession and falls to the ground then he's down,
but a receiver if he loses the ball on contact with the ground in the process of the catch --
then it's NOT a fumble (implying that he previously had possession) it is an INCOMPLETION.
Here is an almost identical play from a year ago -- in which the receiver catches the ball and only do you see on the replay that as he hit the ground,
the ball pops loose for a tiny instant, but in the end the refs got the call right and rules an incompletion.
http://fifthdown.blogs.nytimes.com/2009/09/18/rule-to-review-louis-murphys-non-touchdown/