There is one REALLY long-winded proceedure that isn't even worth mentioning.
However, THE BEST way to slipstream SP1 into any copy of Vista that you have is vLite.
http://www.vlite.net/download.html
You can modify components of Vista; but if you wish to just integrate/slipstream SP1 then it's extremely doable.
...if you'll still read this post, try it out.
r2rX :D
DualBootPRO