Eurogamer has the story. In short:
As part of Blizzard's ambitious plans for the new version of the Battle.net online platform to launch alongside StarCraft II, the company's design chief Rob Pardo [has] revealed plans to allow the mod community to sell content on a StarCraft II Marketplace after launch.The Marketplace would be a forum for publishing and downloading both free and premium maps. "A portion of the revenue" from premium maps would go to the creator
It is unclear what criteria a map would be permitted to become "premium".