I would have thought that it would be highly unlikely.
It's a new console, and given the price point it's coming in at, and dropping the current top model down in price, it shows that a straight replacement for the new 360 isn't a valid exchange.
Their first priority will be to get it repaired. Also, don't forget that just because Microsoft may no longer be making the older Xbox, doesn't mean that stock is going to be totally exhausted. This new model has crept up quite well, I'm not aware that there have been much in the way of reports of Xboxs in short supply, where retailers would clearly be holding less stock because of a new machine coming.
Instead, as they are dropping the price of the Elite, there will still be plenty of stock around for sometime, especially available to large high-street retailers. Then you also have to consider that if it does go back to Microsoft, they still have plenty of reserve stock available to themselves and more importantly, they have the parts to fix damaged consoles.
So in summary, I don't think you are likely to get a new 'slim' Xbox by taking your old one back to the shop, even if it was broken.
