On my MSN Premium account today, I recieved the following email from the Windows Live Mail Beta:
"Thanks so much for your interest in Windows Live Mail beta
This is kind of embarrassing to admit, but we have a bug that won't allow us to make the Windows Live Mail beta available to you at this time. We know it stinks but, if we set you up with this beta, there is a risk that you might lose some of the best stuff that comes with your MSN Premium account. We are working hard to get this bug resolved and once it is, we will send you a mail and move you to the beta.
The Windows Live Mail beta team"
So, it looks like those with MSN Premium accounts looking to sign up for the Windows Live Mail beta through ideas.live.com are out of luck for the time being. Hopefully the Live Mail team will get the issue sorted out shortly.
Update: We have been told that this issue has indeed been resolved, so Premium users who received this email and are not yet on the beta can signup again.