VISTA SP1 : you can't slistream SP1 into an existing install image
BAD news you think , not really : you can't just do an offline upgrade to an image .WIM file because the servicing layer also needs an update.
solution: recapture the image using the latest version of the WAIK
read more about it: here (Kevin Remde)
FYI : VL customers can allready download the vista enterprise w SP1 through the licensing downloads