According to a new report over on PCGamesN, anyone who has prerdered StarCaft II: Heart of the Swarm before December 18th is now flagged for beta access. Blizzard will be letting more and more preorder players into the beta "in waves" roughly each week.
While the full game is set for release on March 12th, the beta access guarantee could be enough of a draw to bring those on the fence about a preorder finally into the yes column. While reaction has had some mixed results so far, Blizzard, s you probably know, is always tweaking things, so trying it out for yourself might be the only way to be sure.
If you've already preordered the game and should have access, you can download the beta client right here.