The information I had was that we built 5584 on Monday, which was intended to be the release. Another internal source told me a single "show stopper" bug (I'm not saying what - but it mattered to Microsoft internal people more than most users) caused them to go back and do one more build. 5600 was built on Wednesday.
By a strange twist of fate my daughter came into work with me today. I made a DVD or 5600 and installed it on a spare Dell: on which she has been watching DVDs and playing purble palace all afternoon. I'm still on 5536 - yep, the only one in this household to have any time on the real RC-1 is 6 years old ! 5536 -5600 is a a pretty small set of changes, so I don't expect much difference when I put it on this toshiba.
I'd keep an eye on this link http://download.windowsvista.com/preview/rc1/en/download.htm for a location of the download. Technet and MSDN subscribers should be able to download it too. Expect it early next week.
Update slight fix to the URL, and a newer build is now here.