On Paul Thurrott's SuperSite for Windows he posted a FAQ for Vista SP1. Here is what he says in one of his comments:
Q: What is Windows Vista Service Pack 1 (SP1)?
A: Previously codenamed Fiji, Windows Vista
SP1 is the first major update to Windows Vista, a collection of bug
fixes, functional changes, and other additions to Microsoft's latest
operating system. As is the case with Vista, however, the feature set
for SP1 has changed dramatically over time. Originally, Microsoft hoped
to ship a Media Center update with SP1 but my understanding is that
this update will no longer be included in SP1.
Q: Any word on when Microsoft will ship the much-need Media Center update?
A:
No, when I last spoke with the Media Center team, I was told that
Microsoft shipped a Media Center update every year around the holiday
season and they would try to keep doing that. At the time, SP1 was
expected by late 2007, so the delivery schedule made sense. Right now,
it's unclear if the Media Center update will ship out of band by itself to Vista users this year or as part of a set of related updates (like "XP Reloaded" from 2005) later on.
http://www.winsupersite.com/faq/vista_sp1.asp