Vista cannot be slipstreamed. Rumor has it that it is due to DRM issues, but it could also be due to other reasons. You either have to install Vista, then SP1 (which is similar to the procedure that Office 2007 does when you "slipstream" SP1 into it), or you need to get a new SP1 .ISO or retail copy.
Pretty annoying for sysadmins, although I have access to the SP1 .ISOs. However, they're not useful to most people because the corporate editions require a Key Management Server to re-validate them at least once every six months.
Intel Q9450 @3.2GHz, Gigabyte GA EP45-UD3P, 4 x 2GB G.Skill @4-4-4-12
Antec P160SW case (modded), Xigmatek 750w PSU
3x 500GB (RAID-5), , OptiArc 7200S, ASUS E818A3T
Creative X-Fi XtremeGamer, Hauppauge HVR-1800, Radeon 4890