I think it's a valid interpretation of the RAW to use an AoO to trip someone again as they stand up.
That being said, the FAQ clarified/ruled/errata'd it such that you are still considered "prone" at the time the AoO occurs, so they get the +4 to hit you, but can't use the AoO to trip you.
I use the FAQ version because:
a) Trip attacks are already powerful enough, and being prone sucks.
b) A lot of people ran it the way the FAQ does, even before the FAQ came out, so I accept that as a valid reading of the rules (just not the one that occured to me when I read them).
c) It makes combat flow smoother, and I can cite the FAQ to settle arguements about it. (Not that the FAQ is totally authoritive, but having a DM ruling backed up by the FAQ tends to settle things faster.)