I tried it as well with a non-flagged account (mine), and sure enough I got in! I guess Blizzard decided to open up the floodgates and let everyone test for the last 2 weeks before they close the beta on the 1st!
That's cool that some people have been able to get in without a beta key. What a pleasant surprise! I was wondering if they'd do something cool like that with patch 17... Enjoy the fun guys! I still remember the feeling I had when I was first playing through back in October, it's pretty awesome.
I hope they just let people go nuts with it and this isn't a bug. It's the end of the beta anyway, there's only two weeks left of it.