I think it's possible to get the 15", though there is a lot of luck involved.
I purchased the uMB in Dec 2008 and the logic board failed a couple weeks ago. I explained to the Apple store how I was in an online class and needed my laptop pretty much at the start of the next week. They would not let me replace it for another 13", but when I told them I'd be willing to upgrade to the 15" they said I could do that if I paid the $150 restocking fee.
So, I got the just released 15", and only paid $250 difference.
Here is how you might have a chance:
1) Talk to the manager, not a floor person. I talked to two managers, and the second one allowed the exchange.
2) In their minds, the aluminum macbook is still very new, so play that to your advantage. They did not seem to care that mine had been 6 months old.
3) IF you want the 15", then this method works. I doubt there is any chance at the 13" because it is cheaper now.
Of course, I was lucky mine had failed. But I still think you have a chance at an upgrade.