Well here is the issue, these values are set at the "market price." Often there is no true market price, not only for retail goods that can often sell below or above MSRP, but even when the IRS has to price any old thing. Technically speaking, if you help your friend move and he gives you an old computer case you have to declare that on your taxes. Now the IRS will never know, and it is of such little value that it is not worth the trouble, but it is either income or a gift (the distinction is often fought and I will not try to get into it here just leave it to say that if there is an exchange of goods and services, in any combination, it probably is not a gift.) The point being, if it were income, and you were going to report it, you would have to come up with a price and that is done by looking at what such an item would fetch on the open market. Hard to judge but you can always find some value even if it is just the scrap value.
With this deal microsoft is giving you software, sort of, and therefore the base MSRP price of the software is a good starting point. However you do not get the bells and whistles that usually come with a piece of retail software, so maybe an OEM price is more likley. Then again, you probably agreed that it would be non-transferable. (Yes, software you buy usually is non transferable but not until you open the box *in most cases*). As an added wrinkle, because the software is theirs and they are pricing it, and they are taking a write-off on it, they want the price to be as high as possible. Boost those profits on paper. This is why it is not a "gift". Because the gift-giver is usually liable for the taxes on gifts (wouldn't apply here for other reasons though.)
So to answer your question more directly- would it be a misrepresentation to claim the full retail price? Probably not according to them, and they set the price. Who is going to contradict them, and more importantly, their lawyers.
so your tax liability is going to be between 10% and 35% of the value they set. see:
http://www.irs.gov/formspubs/a...e/0,,id=164272,00.html
Vista at 65% to 90% off still sounds like a good deal.
Edit: If you really are concerned, and i would wait until you got the form in hand here is what you could do. (This is just an idea, not advice, Get a professional to help/advise). Pay the tax you would owe if microsofts numbers are right, and file a six month extension. Then seek a revenue ruling stating your case. Basically you just write the IRS a letter asking what you should do, how you should value it, etc... They will write you back and their answer is binding for you, not for anyone else. Would not take much time and it may save you some money. In either case you file before the extension is up, and as you have already paid in full (if the ruling goes against you) you wouldn't have any penalties or interest. If the ruling is in yoru favor you get a refund check.
If you have already overpaid and are getting a refund anyway, calculate it as if Microsofts numbers are correct and seek the ruling. If it comes down in your favor file an amended return and get the taxes you overpaid back at that later time.
*Even if that all made perfect sense to you don't do it, go see a professional*