StarCraft 2 is preparing for its third birthday on July 27, which is to say the third birthday of the first release. Blizzard has outlined a few special events in honor of the little tyke.
A post on Battle.net explains that it will be unlocking all three races for the Starter (i.e. demo) edition. That means if you're still undecided on whether to pick it up for some reason, you can try out the Zerg and Protoss armies. You'll also get 25% more XP for a limited time, but the post doesn't specify just how long.