Our sources who have access to some nice Microsoft Websites have confirmed that Microsoft let its volume licensing customers download the ISO versions of Vista with Service Pack 1 inside.
Vista Business SP1 edition ISO is a 2.81GB file and it comes with a few languages, including English, French, German, Spanish and Japanese.
Vista Enterprise Edition 32-bit with Service Pack 1 is about 2.25GB, while the 64-bit version of the same OS is a 2.95GB download, available as ISO.
Our sources have tried Vista SP1 on rather obsolete hardware, but this is definitely something you want to try on some backup machine before you simply install it on all of your PCs. We thank him for his courage; and here are some pictures.