The Windows Vista SP1 image rollout continues and at this point TechNet Direct subscribers can download the Enterprise x86 DVD image. I'm sure the x64 image will arrive in the next day or so. Keep your downloader warmed up.
For those of you with subscriptions, you know there are VL MAK keys available so you can begin some serious testing with the 32 bit platform.
Enterprise has nearly all of the feature set you would ever want, but it lacks the Media Center shell and a couple of other features found in Ultimate.
See http://www.microsoft.com/windows/products/windowsvista/editions/default.mspx for more information on the different versions of Windows Vista.
[UPDATE] The x64 .ISO file is now posted. Subscribers, go get it!