According to BlizzPlanet, Blizzard has not stated officially that Battle.net 2.0 is going to be free.
However, Blizzard is known for its vague comments when it comes to monetization:
Are there any plans to monetize Battle.net 2.0 service? Mike Morhaime limited to answer: “I’m not ready to talk about that today”. Mike Morhaime confirmed that Battle.net 2.0 will be free to those who buy StarCraft 2 retail.
Of course, you need to remember that Starcraft 2 is coming out not in one but in THREE retails boxes (for a total of around $100). So even one measly box gets you into Battle.net 2.0 for life? I’ll take it.