Microsoft are planning a service pack for Windows Vista in early 2008. I've been using Windows Vista for several months now on my notebook and on my work desktop. However, I'm not sure I see the need for a service pack since Windows Update installs hotfixes as they are released. The service pack will rollup all the hotfixes and provide minor enhancements to system software such as BitLocker and Defragmenter.
Paul Thurrott has written some articles about the service pack on his site. He seems to be of the opinion that Microsoft is co-releasing the service pack with Windows Server 2008 to encourage business users who typically wait for the first service pack before buying Microsoft products. I can understand that perspective but must point out that Microsoft have become much better at delivering quality software.