Only Microsoft knows for sure, but they've been trying to get away from releasing service packs for a while now. First they cut short SP5 for Windows2000, then took their sweet time getting XP SP3 out, and odds are SP2 is the end of the line as far as major updates go for Vista. Doesn't mean they won't still be releasing individual updates for many years to come, but they just won't be packaged into a handy little package that system builders can use to get to a certain baseline without having to download a bunch of crap off the Internet.
Of course I can see some benefits to Microsoft's way of thinking. It's a huge resource drain to develop these things, which doesn't make Microsoft any money. They have to create new install discs that incorporate this service pack. And, to top it all off, it's an even bigger effort to try and get people to install them. You still have plenty of morons who insist that service packs break their system, when it's more likely the hundreds of different malware programs crawling all over their system that did the damage. There are exceptions, like the boneheaded move HP made using the same image on Intel and AMD systems that caused all kinds of fun for people who've already suffered enough by nature of owning an HP system. Then they had to have a serious insult added to that particular injury.
I wouldn't completely rule out the possibility, but I think they're trying to break people of the habit of expecting them on some kind of regular basis. They might put out a service pack now and again, but it will be more on a basis of need, and pressure they're getting from their multi-million dollar contract holders.
I have heard that there won't be a service pack 3 for vista. Is it true?