June 28 UPDATE: It looks like we've found the solution!
A lot of people have been reporting problems logging in after installing the MSN Messenger 6.0 beta. Rather than create a million new topics that repeat the same thing, I'm going to collect the best answers and put them here.
First, before you install (or reinstall) MSN Messenger 6.0, make sure you've completely exited any prior versions. Closing it or signing out isn't sufficient. Right-click the little Messenger icon in your system tray and click "Exit." Then it'll be completely shut-down.
If you still can't sign-in,
do not use the sign-in button. Instead, click the link below that which reads "To sign in with a different account, click here." This will open the .NET Passport sign-in screen.
Re-type your password, even if it was previously saved. I've found that this will work with accounts that won't work otherwise. In fact, I have two accounts that I use. One works fine with the password saved, the other
never works with the password saved (even though it used to with 5.0).
If that doesn't work, try another account. If you don't have another account, create a new one and try that.
If it still doesn't work,
re-install MSN Messenger 5.0!