Yep. Apparently Lil' XT has gone missing from the Blizzard Pet Store. A few other collectors noticed and the questions began on the forums. It sounds like it's only temporary, but details are sketchy. So far, here's what we've got from a forum blue:
Lil' XT has been temporarily removed from the store, everyone. We plan on bringing Lil' XT back as soon as possible, though. While I don't have an ETA to provide, I encourage you to check back over the course of the next couple weeks. :)
Based on this statement, there may have been a system glitch. Or maybe more codes need to be generated. Either way, we'll keep you posted.