From a recent Latin America press release:
"With this purchase, players will have six months of access to the StarCraft II single-player campaign and multiplayer play through the new version of Battle.net®, the premier online gaming destination for Blizzard Entertainment gamers. Continued StarCraft II play beyond the time included with the initial purchase will be available via 30-day and 60-day subscription options. Pricing for these options will be announced at a later date."
Looks like they are testing this out in Latin America... won't be long before this is standard for all Blizzard games worldwide.