If you want a beta key for HoTS apparently you get one after pre-ordering HoTS from amazon. I'm guessing you can cancel your pre-order after getting the key too. :-P
Don't hold me to the cancel though. I don't know if that works.
"Beta Access for Starcraft II: Heart of the Swarm Pre-order Starcraft II: Heart of the Swarm and get beta access. We will email your code within 4 business days after you place your pre-order for Starcraft II: Heart of the Swarm Standard or Collector’s Edition. Offers are valid when shipped and sold by Amazon.com. This offer will be extended to all existing pre-orders. Limit one per customer. Amazon reserves the right to change or terminate this promotion at any time. "