Gearboxity has a follow-up to the news from PAX East about plans to add a fifth playable class to Borderlands 2 via post-release DLC. This clarifies that this Mechromancer content will be a preorder bonus for the role-playing shooter sequel, and paid content for everyone else:

As a special thank you to our early supporters, the Mechromancer will be available as a free download to members of the Borderlands 2 Premiere Club, which you can gain access to by pre-ordering Borderlands 2 from a participating retailer. The Mechromancer will also be available as premium downloadable content for those who aren't Premiere Club members at the same time. For more details on the Borderlands 2 Premiere Club, please visit this thread and borderlands2.com/preorder.