Boy, I'm sure glad I don't work at Celebrity right now
I've asked, but haven't had any official yes or no. A cruise line doesn't intentionally string passengers along who are booked. It's a matter of learning as much as possible, projecting what can and cannot be finished in time, when consulting with the ship yard.
If the ship is delayed, passengers will get a full refund and probably a credit towards a future Celebrity cruise. Of course, that never replaces a person's scheduled vacation time, all the problems involved in finding another cruise.
Regarding the delay in Millennium technical problems -- which may affect Infinity as well, since she's a twin -- the problem happenened during Sea Trials. During Sea Trials, the builders, owners and insurers of a ship take the ship "through her paces", so to speak. They test the ship in every way possible -- full spead ahead for eight hours, full speed backwards, circles, etc. The one thing they didn't do with Millennium, however, was test the ship at very slow speeds. Otherwise, Celebrity wouldn't have taken possession of the vessel. Here's where the technical glitch wound up.
It's a good company that's had some tough breaks lately. If there is any delay, you'll find it immediately in CruiseMates' news area on the home page.