After being on the phone twice with Apple's tier 2 support, I believe the solution is the $99 mini displayport to DUAL-link adapter. Problem is, I can't verify this as they will not be available for 4-5 weeks according to Apple's website.
I am having this same issue with my new MB but not my Mac Mini. After further testing with both system, it seems as though the absence of the "overscan" feature has to do with the detection of a powered USB cable connected in conjunction with the connected DVI cable.
The new $30 adapter from Apple lacks this USB input signal, but the more expensive adapter has it integrated into it via an additional, second cable.
Sad part is, even though both of Apple's tier 2 support specialist agreed I needed this more expensive adapter for this to work, neither knew the reason why. I only figured this out with some common sense and trial and error.
I do hope this helps those looking for an HD solution. Please feel free to PM through the MR messaging system for any clarification should you need help understanding the above and why it works with one and not the other.
Again, I'm not 100% positive, but believe this to be the solution.