In the most recent podcast they said they were holding off on releasing it for "a few more weeks" to make some changes to it.
It makes me wonder if it was rejected from the App Store and they have to do revisions to get it accepted. I was surprised when they initially gave a specific release date, because the App Store approval process is notoriously fickle. Things get rejected for strange and often unpredictable reasons. I've never heard of any developer knowing for sure when their app or updates will be approved (look at Google Voice, supposedly still being "reviewed" after months. Of course that is a special case, now involving the FCC. I doubt Project X will be that controversial!) I follow the developers of some of my favorite apps on Twitter, and they are always talking about how their updates have been submitted and they have no idea when they'll be approved.
Hopefully Project X will make it through smoothly and quickly because I'm excited to try it out!