The application for the release candidate requires filling out a simple survey that was only 10 questions for me. The beta-testing program has apparently been quite popular, so I'm not sure how much longer the application process will be open. According to the program information for Windows Home Server RC on the Microsoft Connect site, the "(b)road Public Beta RC is setup to track all of the people who join WHS after the RC Milestone including the 25k applicants who applied after CTP (Community Technology Preview)."
I applied for the new release candidate yesterday, and haven't seen anything in my e-mail yet, but a message on the Windows Home Server page yesterday said that all applicants would receive instructions on downloading the beta release within a week. However, that message is no longer on the page, so I'm not holding my breath.
Along with the new release candidate, Microsoft is also promoting the imminent server software with a contest for developers of both software and hardware add-ons for Windows Home Server.
Named Code2Challenge, the contest includes over $50,000 in prizes, including $10,000 prizes for the 1st place finalist in each category. If you're thinking about entering, you should definitely start with the Windows Home Server SDK, available via the Microsoft Developer Network.