Rojo, go look at the slow-motion video "common" has posted (http://www.dailymotion.com/video/xsqnz4_fall_sport). Then consider retracting your request for a re-run. If you really want them to re-run this race, then you'd have to also want to re-run any race where someone fell.
As many others have pointed out, Uceny's left trailing foot makes contact with the Ethiopian's right thigh/knee, so when Uceny's left foot goes forward it hits her own right calf and she goes down.
I think that much is beyond debate.
The question of who is at fault for that contact is a matter of opinion. Personally, I'm inclined to say that it was incidental contact and nobody's fault.
But even if you want to say it was a foul, it was an extremely minor one - at worst, the Ethiopian shifted her position very slightly into the path of Uceny's trail foot, a matter of probably a few inches at most.
I can't think of a lesser foul, really. The Ethiopian wasn't fighting for position, cutting someone off, or using any aggressive racing tactics. So if this race merits a re-run, I can't imagine how someone could fall in a race where you'd say it didn't merit a re-run.