For reasons that are as of yet unknown, Microsoft has opted to push back the release date of the anticipated second service pack for Windows Vista. Now, Microsoft is apparently preparing to ship a Release Candidate “Escrow Build” for the service pack in February, and assuming all goes well will release an actual RC in March and then there’s an RTM scheduled for sometime in Q2. This is quite different from their original plan to go final sometime in April. Once it is released, it'll be a staged rollout, so SP2 could be several months off.
The official release date for Vista SP2 is up in the air, with no official date set – but it could be later than Q2 altogether, as the current “release to web” time is unannounced. This could be the result of findings from their service pack testing, which was expanded at the end of last year. If problems have been discovered, Microsoft has been pretty quiet about them. It's not terribly surprising, as we've used to service pack release delays and many have suggested that it will arrive no sooner than April anyway.