You can now download the opt-in tool from Battle.net to check your system spec and send it to Blizzard. This will sign you up for the SC2 beta that will start sometime this summer.

You will need a battle.net account with at least 1 game by blizzard, so go find the CD key for your old Starcraft, Warcraft or Diablo copy (it'll give you access to the digital download version so it's win/win) and get the beta opt-in tool here.

edit: I forgot to mention that WoW will also work, for those of you that play it.