According to Wzor Microsoft has started working on Windows 7 Service Pack 1 labeled as 7138.0.090523-xxxx-winmain_win7xxx_sp1_x, though compilation date cannot be found as build number is blocked in a screenshot posted by Wzor.

There is no idea why Microsoft would start working on Service Pack 1 of Windows 7 so early and it makes no sense of having service pack before actual release of Windows 7. Means that Microsoft is in process of implementing features those will be left out from final release of Windows 7.

I think it may be another marketing strategy of Microsoft to popularize Windows 7 in the public.