Add a 30" this year to the iMac refresh and include an option to add an internal AppleTV. I plan on replacing my original G5 iMac (ordered the day it was announced) with a new one (to be ordered the day the Sandy Bridge is announced) and I would pay for a 30" upgrade, especially if AppleTV is included.
As for the $3.9 Billion, I can see displays as being at the top of the list. I can also see Apple being willing to invest more (up to $10 Billion) for other enhancements, like doubling the size of their new server farm, enhancing protection for other components for their mobile lines (including capital investments capital equipment for producing memory.
What else would Apple potentially need that would benefit from pre-payments.