The Windows SDK team has now released their September CTP to match the Windows Vista September CTP. While Windows Vista September CTP is only available to beta and TAP members, you can use the Windows SDK on "downlevel" machines - Windows XP SP1 or Windows Server 2003 (SP1 or R2). You can find all the links you need for the Windows Vista and .NET 3.0 development tools in my Compatibility Matrix blog post.
Speaking of that matrix, I wrote that blog post a couple of months ago (when July CTP released) to try and mitigate the confusion caused by having three concurrent CTPs/Betas of Windows Vista and the various .NET 3.0 development tools. The scenario of having multiple supported CTPs/Betas was an anomoly so I didn't anticipate updating that blog entry. However, the matrix has proven so popular that I've decided to continue updating it through RTM with each of our Windows Vista and .NET 3.0 development tool releases.
Windows Vista, .NET 3.0 and Windows SDK Compatibility Matrix