A quirky addition to Diablo 3's patch 1.0.3 is the purchase of a digital copy of the game will restrict players to the Starter Edition for "the first 72 hours (sometimes less)." This means legitimate customers can only play Act 1 up to the Skeleton King, cap at level 13 and can only interact with other Starter Edition players.

We could easily speculate why Blizzard would put such precautions in place, but it doesn't ease our concerns about hostile actions against legitimate customers. Oh, also, given the wording on the post, this is a feature, not a bug.

Take this to mean if you're looking to impulsively marathon Diablo 3 over some upcoming free time, you'll need to pick up the game from retail or overnight a boxed copy.