I'm not too sure what's happening, but it seems that an unusual influx of people have gotten invites to the HotS beta (going from this Reddit page) and it might be a Blizzard mistake. Checking my Battle.net account, I'm inclined to believe I'm one of the people, but I'm not sure and the client is currently getting patched for me so I'll know after it finishes.
Are you one of those who can now access the beta?
Actually, it seems that people are also experiencing region issues that are not allowing them to gain access to the beta, so a bunch of weird stuff is happening. I just tried the client out and it says that I have no account associated with whatever region it's talking about, either.