The folks at NeoWin have a great FAQ about a lack of slipstream capability with Vista SP1.  Read more here, but the bottom line is there's a "servicing layer" in Vista that allows for easier application of most updates, but SP1 includes updates to this layer that prevent a WIM image from being updated offline.  The solution is to build a Vista image, apply the new service pack, sysprep the machine, then take a new WIM image.

That or wait for Microsoft to release a new DVD with the SP1 bits already added...

Also note that a new version of the Windows Automated Installation Kit (WAIK) is available for Vista SP1 and Server 2008 here.

Update 18:25MST - WinBeta.org has an interesting write-up of an unsupported but potentially workable SP1 slipstream using SysPrep in their Vista SP1 Reverse Integration Guide.