Theres a few articles on the Internet indicating that Vista will not have any Service packs, in there place are smaller updates being shipped via Windows Update. Because Vista is so well done its seems that service packs are not required, mainly because no major changes to the OS are needed. At first i was a little shocked and concerned about this, but after thinking about it i can see why Microsoft is dumping its traditionally methods. Here are some benefits over updates compared to Service packs.

Faster release

updates can be done separately, smaller updates may be released sooner, rather than waiting for the next service pack.

Choice
Unlike Service Packs, You can choose with updates you require canceling out any unnecessary installs.

History

You can look back on what updates you have installed and uninstall any that cause any problems.

So my final thought on this topic is, Windows Update is expanding and doing all the work services packs do, but in many ways better.