Power --
It may not be a requirement for Honda to make the Pilot as quick (acceleration-wise) as the MDX. After all, the MDX has better acceleration than all of its direct competitors to begin with. Thus, even if the Pilot is a couple of hundred pounds heavier, they can put the MDX engine -- or, more likely, the Odyssey engine -- into the Pilot and get a good-for-the-class acceleration.
Dave --
Ouch, I'm very sorry about munging your real first name. For some reason that's what stuck in my head for your initials.
Yes, speculating isn't quite as fun when you're not really interested in buying the vehicle (not that it's not going to be a good vehicle, but like you said, we got our MDX's now!).
I agree that the differentiation may not be very large. When the Acura TL was introduced its differentiation from the Accord EX V6 was primary in the trim, slight body differences, more power, the sportshift, xenons, auto climate control, and available nav system. There were a lot of arguments over whether to go with an Accord EX V6 or a TL, especially back then when the TL commanded MSRP.
The Pilot will have the different trim, body differences, quite probably different power results, and just be plain bigger. It may be that Honda will consider that enough to start with, and add upgrades to the MDX (e.g. the rumored upgraded engine to give it higher performance, maybe xenons one of these centuries, maybe VSA, etc.).
I'm not sure what else Honda would do to differentiate it. Dropping auto climate control wouldn't make sense because the Odyssey has it. Dropping the front/rear individual separate controls would be bad in a vehicle even slightly bigger than the MDX, but that's a possibility to cut its cost (e.g. just put vents in the back).
I suspect what'll happen is that the price difference between the two won't be huge, and there'll be countless threads debating which is the best value. Unfortunately for a lot of people (but not all) the difference between Honda and Acura dealership service won't be as pronounced as the difference between Toyota and Lexus.