Lead designer Dustin Browder revealed at a Valencia eSports Congress panel that Blizzard is "looking at free-to-play as an option for multiplayer." However, at this point, Blizzard isn't sure how to go about doing so, since asking players to "pay-for-units" goes against the unit balance that the series has created.
Obviously, adopting some form of a free-to-play model would only benefit StarCraft 2 and Blizzard, especially in the world of eSports. It will definitely happen sooner than later, but of course, they want to make sure they can do so without losing an important part of the game.
How would you recommend they make StarCraft 2 free-to-play?