It's a state-by-state statute. In NC, it's illegal for there to be an open container of alcohol (which means one not sealed in its original container, so even transporting a corked half bottle of wine to a party or something) in the car if the driver has any detectable amount of alcohol in his system, even a level that wouldn't otherwise be illegal.

If I remember correctly, if it's found that the driver is not legally drunk, then it's basically the same as "public drunkenness" and is just a minor fineable offense. Mostly, it's intended to make drunk driving have a stronger penalty, and, as such, is absurd.