It seems another wave of invites for the WotLK Classic beta have gone out, as players are reporting receiving access a little earlier today. It also seems there aren’t any mails sent out to notify players of their beta access, or at least they aren’t arriving as fast as the invites themselves, as the game just shows up as an option on the Battle.net launcher. It seems a fair amount of those invited were active in Burning Crusade Classic, as sethers656 reported 10 players in their guild getting in, although some that don’t even play Classic at all also received an invite.
So check your WoW section of Battle.net and see if you managed to get in as well.