Blizzard should be kicking off its Diablo III beta test any day now (it's currently in the "Friends & Family" phase, which is right before the floodgates open), but you're impatient, there's a solution.
The company has launched a "Skill Calculator" online, which basically allows you to start rolling a character ahead of actually having to spec one out in the game.
In essence, the addiction can start early. Way early. It's also incredibly mean, given the beta test hasn't started yet, though one could argue it will be even meaner for the players who never get into the beta.
If you haven't had a chance to look over the skill tree for Diablo III, now's a good time to brush up. If you haven't picked a class to jump into yet, here's an easy way to become familiar with the new ones.