Memory manufacturers prefer money. That's all they want. They don't care who is buying their products as long as they pay and as long as they can sell ALL thier STOCK. Since they have low stock and they don't have high production right now, ANY company can buy that amount, so they would sell it to the one that paid more. Could Nvidia pay more than AMD? Maybe (well, sure), but why would they want to do so? It would make their cards more expensive, but what is worse for them is the REALLY LOW AVAILABILITY. Let's face it, Nvidia has a 66% of market share. That's twice of what Ati has. If availability is low for Ati, much more for Nvidia. Contrary to what people think, I don't think Nvidia cares too much about Ati and a lot more about their market audience. GDDR5 would make their product a lot more expensive and scarce. They don't want that. Plain and simple.
And HD4850 WON'T have a GDDR5 version from AMD. They gave partners the choice to use it. That way partners can decide if they want to pay the price premium or not. GDDR5 price is so high, that AMD has decided is not cost effective for HD4850. Now knowing that it's only an underclocked HD4870, think about GDDR5 and tell me in all honesty that it's not just a marketing strategy.