The simple detail is you can not run Exchange 2007 on Windows 2008 R2 until 2nd half of 2010. Exchange 2003 however is good to go now.
They must really have screwed up Exchange 2007.
Later in the post comes this request, which most people have asked for from the start:
Finally I do want to update all on one other piece of feedback we have received - allowing the in place upgrade of the operating system under Exchange. Technically the work required to provide this capability is consistent with the work we would need to do to support an in-place upgrade of Exchange itself. As such the amount of work needed is outside the scope and complexity of what we can do in a post release product update. Still we do understand the demand and desire and it is something we will continue to look at for future versions of the product.
In other words, too bad, we don't care, go buy new hardware and pay your Microsoft Business partners more money.
Sometimes I wish IBM would do this, then I remind myself that while it would make us more money, it also makes life much harder as we would have to convince customers to upgrade which as can be seen, if there is such usage of Windows 2003 still, the Microsoft faithful don't sound so faithful do they?