The Starcraft 2 demo is finally available for PC and Mac. The sample includes the first three campaign missions from the full version It also has a few skirmish maps that will let you play as the Terran faction against Terran AI opponents. The demo doesn't require a Battle.net account (so no online, sorry). It's perfect if you're still debating relinquishing all of your free time to the game.
Have a looksie here.