This is no doubt fallout from the just ended Hollywood writers' strike, which lasted long enough to seriously impact the movie and TV industry. There has been no work done on screenplays for almost three months, and that will have a domino effect that will be felt not only this year but through 2009 as well.
At the moment, the 2009 movie release schedule is looking pretty thin, and part of Paramount's rationale is that Star Trek might be able to make a lot more money if it opens during the hot (no pun) summer movie season than during the Christmas holiday.
No doubt we'll be seeing more release date shifting over the coming weeks.
This may also end up helping the quality of the film, depending on how much has already been shot, because now script changes can be made during filming. Due to the strike, up until now due to the rules of the system even if director J.J. Abrams came up with an idea to change a scene, he couldn't use it since the script couldn't be changed.
On the other hand, I'm bummed about this because now we'll have to wait an extra five months before seeing the movie, and I anticipating it quite a bit.
Star Trek now opens on May 8, 2009.
Source: Variety via TrekMovie.com