Heart of the Swarm will be released in a mere three months except for selected beta testers. Blizzard has just recently padded that roster with any Battle.net account used to pre-order Heart of the Swarm before December 18th.
For those of us who have yet to pre-order, Blizzard will continue to add pre-order customers to the beta program in waves. There is no information on how frequently the developer intends on adding fans to the beta although I would expect that ordering soon than later would be a wise idea.
You will not be able to join with any old pre-order, however. Only certain retailers will allow access to the beta. Purchasing direct from Blizzard is a very stable way to guarantee access while customers of other retailers need to ensure that their retailer of choice advertises beta access. If you do not see claims of beta access, it is pretty certain you will not get it.