I have the late 2008 Macbook (aluminum Macbook) and I was just thinking... What will Apple replace my laptop with if it breaks after the parts for this Macbook has been discontinued and unavailable? A plastic Macbook or a MBP? I supposed that they will replace it with the closest model that is be available at the time; the definition of "closest" is the question here.
They still have an inventory of the MacBook. But if they run out, it's like what others have said, will replace with a MacBook Pro with something very close to what you currently have.
In my experience, Apple's replaced whatever's broken with whatever's comparable and current, so if yours were to crap out, you'd get something comparable today. But that's just what happened with my old MacBook and iPod, so YMMV. Honestly, if you're rocking an aluminum one, yours is recent enough to not really worry about it going anywhere soon. The plastic MacBooks, on the other hand, will be going the way of the dodo within a year or so.
sometimes they just give you a credit for whatever your macbook's MSRP was, and you can pick another mac with that credit. You have the option to pay the difference and upgrade to a more expensive macbook.