Zerg fans, be happy: the Starcraft 2: Heart of the Swarm beta has just begun. This one includes the multiplayer portion of the game only, and is open to both PC and Mac gamers.
The bad news: it's open only to a select minority of players for now, and by the sounds of it, anyone who tested patch 1.5 or is a pro gamer or part of the press gets priority, as do active StarCraft 2 players. Naturally, Blizzard will be offering it to more people as time goes on, and various sites will be getting keys for giveaway. However, even if you don't get in, there's no NDA so you can whet your appetite some by checking out the various streams and recorded footage around the interwebs.
If you're the Zerg rushing type, make sure you're opted into the beta by checking your Battle.net "Beta Profile Settings" under the Account Management section -- this will give you a shot.