just got mine a week ago.. sign up on battle.net and apply to the beta. I also added warcraft3 that I had laying around on my battle.net account and maybe that helped because I got the beta the day after :p
did you add some game to your battle.net account? warcraft3, diablo, SC1 or something? I think battle.net don't send out invites to people who haven't got another game on their battle.net account because I've heard alot of people who have making lots of battle.net accounts and never got an invite. I got one after just like 1 or 2 days after adding WC3 to my batte.net account.
haha :)