"Can I slipstream Windows Vista SP1 into an existing install image?"
Nope. Well, not directly, anyway.
"Um.. but when I get SP1, I want to upgrade my deployable .WIM images with the new bits. I can't do that in an offline way like I can with other updates?"
"Are you going to tell me why?"
Absolutely! You don't think I would have opened up this nasty can of worms without giving you a good explanation, did you?
Okay. So here's the deal. And those of you who have experienced the SP1 installation have experienced this as well. When you do the SP1 installation, even if it's from Windows Update (when available), you're going to see your machine shutdown and restart on it's own several times. That's to be expected. See, there's this important part of the OS known as the "servicing layer" in Windows Vista and Windows Server 2008. This is the part of the OS that allows for easy update installation with minimal disruptions, allows for an update to be applied to an offline captured image that's within a .wim file, among other things. Well... let's say that that servicing layer ALSO needed to be updated? What then?
"Oh.. I get it. You can't update the thing that makes the updates happen smoothly, because the thing that makes updates go smoothly is itself being updated!"
Bingo. You got it. So hopefully the news that you can't just do an offline upgrade to an image .WIM file won't be too tragic.
"So.. what do I do instead?"
You are going to have to install your image to a machine. Install the Service Pack. Then re-capture the image.
Not so simple. There are additional steps that involve some cleanup once you've sysprepped your newly updated SP1 machine. Detailed steps are available in the new WAIK documentation.
"Won't I lose a valuable re-arm to my image when I apply the service pack this way?"
No. SP1 grants you an additional re-arm. We don't want you to be penalized for having to generalize a system that additional time.
"Wait... there's a new WAIK?"
If you're using the WAIK (Windows Automated Installation Kit), you will definitely want to get the new version that has support for both the original Vista as well as the new servicing layer that's in both Windows Vista SP1 and Windows Server 2008.
Of course, the easiest way to do this all would be to get a copy pre-slipstreamed SP1 version of Windows Vista from Microsoft when it becomes available, and start with that as your new installation base. If you're not doing any other custom image management, that's definitely the easiest solution. Just add it to your own Microsoft Deployment workbench or use it to build your new images from there.
On other notes, Microsoft made its own slipstreamed image available to its MVLS partners!