What you need to do to get a Vista Release Candidate 1 product key is the following: Visit the link to the Microsoft Vista homepage and sign in. If you have not signed up before you can easily create a new account by clicking signup now. You only have to provide some information, a working email address, a password and the like. Microsoft sends out a confirmation email at the end of that process.
If everything worked out fine you should be directed to a website which requires some information about your current job, filling that out brings you to a new page that links to vista downloads (English, German and Japanese versions) and displays the product key in the same window. One funny thing that I noticed is the following: If you refresh the page with the information the product key stays the same, if you open the page using the link above you will recieve a new product key. If you need more than one I guess you can just visit the website a few times.
The website with the product key looks awefull in firefox btw.