Microsoft announced last year that SP3 for XP would be delayed in lieu of developing a SP1 for Vista. They've now given us a tentative release date
for Vista SP1, which they say will be arriving in the second half of this year. They are offering people the chance to test the service pack early, as they do with more of their software. While some may think of that as being too soon, it actually is pretty normal. With businesses already installing and running Vista, if Microsoft should release a service pack towards winter, it'll be nearly a year after initial availability, which is the same time that they took for XP SP1.
They won't give an exact date yet, of course.