How likely is it that Apple updates the design in 2012?
Likely. I wouldn't necessarily bet on it, but likely.
If they had a MBA with 8GB of RAM, I'd be all over that. We'll see. If the next MBA refresh offers that much RAM, sold. If not, I think I'm gonna wait one more refresh on the MBP. My current machine does everything I need. (Although... I would love a quad core in my 13" for VMs. My use is edge case though so I'm certainly not expecting it.)