Three scenarios have been discussed so far. The problem is that there are holes in all of them. So one will have to either go with the scenario with fewer holes, or just wait for more information. These scenarios are based on a couple of established facts:
1) That Amiri was involved in the Iranian nuclear program
2) That he disappeared in Saudi Arabia
2) That the CIA had him for a while (as confirmed by the American authorities back in March), either by his own will or not.
Now to the scenarios:
Scenario 1) That the whole thing was a clever scheme by IRI to send someone with some nuclear information (but nothing too sensitive) to either defect to US, or act as a bait to be abducted. In this scenario the US gets excited thinking they have caught a big fish, while the IRI plays for two objectives: 1) a PR victimization show to the world, 2) gaining knowledge of CIA/Saudi intelligence internal working, and 3) more importantly, to make it unlikely that anyone else can really defect because now the US authorities will be highly suspecious of anyone claiming to defect from Iranian nuclear program or military. I think this scenario is very probable.
Scenario 2) That Amiri was really abducted by Saudis and handed over to Americans, where he may have given them some information voluntarily or under force. before totally rejecting this scenario, remember that the "rendition" plans that became common under George Bush. It is possible that once the CIA got all information Amiri had, he became irrelevant to them and thus he was given the freedom to go back to Iran. Obviously he has to make it more dramatic (posting messages, claim of escape etc) to avoid being grilled by IRI back home. This is also a probable scenario, though if true, one wonders why Americans did not choose to charge him for violating UN non-proliferation laws.
Scenario 3) That Amiri defected to the US, but because of IRI pressure over his family, he decided to go back. While it is a possible scenario, it is also the weakest case. First of all, someone in such a sensitive post would not defect without the knowledge of what would happen to his family. His return to Iran in this scenario will mean certain death as spy; why would he want to commit suicide after taking all the troubles to defect in the first place? and third, knowing all the above, the CIA would be grilled for letting a genuine defect go back to the sharks. they would never allow that, especially considering the information and experience Amiri may have gained from his CIA contacts. I would not think this scenario is very likely.
My 2 cents.