MS has started working on Service pack before releasing final product. Many people are baffled how it could be - still at full speed the development of Windows 7, but it has already started Service Pack 1. People have no idea why Microsoft would start working on Windows 7 SP1 so early and it makes no senses of having service pack before actual Windows 7 release. Explanation could be one - apparently because of lack of time, some important functions fail to incorporate the core, and they will join later with SP1.
What specific functions will satisfy SP1 - not yet known.Maybe it is another marketing strategy of Microsoft to popularize Windows 7 among users. Windows 7 is on the other hand a stable operating system that should run fine even without the service pack.