That was different. The Nexus line always has successors, and each phone pretty much pushes the last one out of the limelight. I'm pretty sure they weren't selling the Nexus S in the Play store for long after the Galaxy Nexus came out.

The Moto X is more of a competitor than a successor, so I don't think it's very clear what Google will do with it when the X is in the Play store.

Frankly, I'm a little frustrated by the lack of information we have about the release plan for the X. We don't know when it's available anywhere, do we?