In particular, the the dual GPUs in the MacBook Pros can apparently be run in a Hybrid SLI mode allowing them both to be active at once (and thus increasing graphics performance over the discrete chip alone). In addition, the hardware is apparently capable of on-the-fly switching between the two GPUs instead of the present implementation which requires logging out to switch between the integrated 9400M and the discrete 9600M GT. In principle, this would allow the machine to dynamically switch between using the discrete chip when plugged in and the integrated chip when running on battery power.
So what's the upshot? In the short run, not much. Just because the features are supported in the silicon means squat unless Apple decides to implement them in software. In the best case scenario, however, the new MacBook Pros might see performance increases with only software and/or firmware updates. Of course, whether Apple will choose to do that (which would perhaps decrease the incentive to buy the next generation of machines) is another question altogether.
Apple MacBook Pro 15-inch with Retina Display (mid 2014)