Reading B.net forums.
This is on Starcraft though, hopefully this will be the same for Diablo 3.

"When a player buys the StarCraft II box at retail, they will have the ability to play on the new Battle.net for free.

For those listening to the latest Activision Blizzard conference call, Mike Morhaime also mentions it there as well."

http://forums.battle.net/thread.html...=3000&pageNo=2

That's by community manager, Karune, middle of the page.