While plenty of people are willing to go out to a store or purchase a boxed copy online, many more turn to Battle.net and quickly pick up a digital copy. As of the new 1.0.3, though, new customers can apparently only unlock the full game three days after purchase.
In a post on Blizzard's support forums, the details are made clear (though not explained at all):
As of patch 1.0.3, when purchasing a digital version of Diablo III through the online store or your Battle.net Account, players are restricted to the Starter Edition for the first 72 hours (sometimes less). Players on Starter Editions have the following restrictions:
-Act I up to the Skeleton King is available
-Level 13 cap
-Matchmaking available only with other Starter Edition players
-No Auction House access (Real Money or Gold)
-Global Play is not available. Players attempting to connect to Diablo III Starter Edition in a region other than their Battle.net Account's home region will receive Error 12. See the Global Play support article for more information.
Basically, people who go out and purchase the game in-full are restricted to the content from the Beta.