http://download.windowsvista.com/preview/rc1/en/download-5728.htm

or download the ISO directly from Microsoft

The link worked for me and I am not an “official” beta tester. I did register and download Beta 2 and RC1, so if you have your product key from those builds - go for it.

Here’s the disclaimer from the page on Microsoft’s site.

This page includes information to help you download a special test build of Windows Vista. This build (5728) has a number of improvements and updates from RC1, but has not been put through the same internal testing process as RC1 and therefore may be unstable in certain installations. We are making this release available for a limited time only (and only by download) in order to get broad distribution and testing in a variety of PC configurations. Please note: This build may not have the same level of support or servicing via Windows Update, and you may not be able to upgrade to the final version of Windows Vista.

Source: In House