It's interesting that it was Anthony Ervin who got it right. When Phelps was adamantly declaring that his retirement was going to be permanent, Ervin was predicting that it wouldn't be. I'm pretty sure that the reason Ervin was willing to disagree with Phelps about what Phelps was going to do was because Ervin, unlike Phelps, knew what it is like for a competitive swimmer to try to stop competing.
Phelps undoubtedly wanted, and perhaps even needed, a break from the demanding and downright oppressive training schedule that an Olympic swimmer has to follow. But Ervin knew that, for a competitive swimmer, the training and preparation, and even the competitions themselves, have become such an integral part of his life that he will inevitably feel a vacuum if he tries to give it up.
Now, of course, Phelps has a wife and son to occupy his time, so there's more of a chance that he will retire, at least for awhile.