From what I've seen, stash is right. You must have an activated beta 2 key to get RC1. I've been putting off installing Vista for a while now even though I keep telling myself to do it. Here's hoping it's not too late.
Anyway, you need to have signed up for the beta to get a valid key. If you did that, but don't have the ISO, then you'll have to use bittorrent to get it.