If Amy Pond was a trap, laid so that the Doctor would be directed to the Pandorica, who could have set it? Someone who knows how the Doctor chooses companions, someone who could sabotage the TARDIS to arrive twelve years late ... someone who could fashion an alliance among all the Doctor's enemies, possibly someone who could take control of the TARDIS while River is in it. Who could do all that?

The Doctor himself? River Song? Amy's "aunt"? Another Time Lord? Some combination of the above?

Further idle speculation: When the Doctor is telling Amy about how people fall out of the world sometimes, I get the odd feeling that it was actually Moffat telling us the answer. People fall out of the world sometimes, but they never disappear completely. They leave bits and pieces behind, things you can't account for. What if someone fell out of the Doctor's world and he can't remember her? She would have to be part of his timeline for that to work, ie. a Time Lord. (I say she because that's what Amy says when she hands the ring back: "She must be nice, this friend of yours." )

There's a lot about that idea that doesn't make sense either. But at this point, a handful of days before the finale airs, I'm literally grasping at speculative straws. The person who vanished would have to have returned already via being remembered, or will be returned, or something. There would also have to be clues in the season -- bits and pieces that you can't account for. So, yeah, no idea.