Only Apple know what Apple will do but other than a Spec bump or two I seriously doubt you are going to see new designs. After all the current designs are fairly new and pretty darn good.
But they need to maintain separation, they aren't stupid enough to make a product which will just steal sales from the expensive macbook pro lines.
It'll also be difficult to make cheaper. Can't cut the RAM, can't realistically make the CPU slower, can't change the graphics (integral part of the chipset). But the unibody enclosure is very expensive so killing that off is one option - I can't see them making the macbook unibody - there's no way to differentiate it from the macbook pro then...
So what would they do with the WhiteBook then? The design is good*, the CPU and graphics are good and the position of ports and so on is good. The screen isn't brilliant (at least not on my 2007) but is good enough.
So what can Apple do? Can make it more expensive as it would compete with the Pro sales and I don't see Apple reducing the costs. Apple don't sell cheap computers.
* As long as they fix the plastic peeling problem.
My basic point - those tightwads are increasing in number and apple has to either sell a product in that market or convince them to spend more - a tough order but if any product line is suited to the task it's the macbook and it's due a refresh now anyway.
Sorry i didnt make that clear, i meant the same innards in the sense that they can share a lot of the components etc, just obviously a lower-spec version. But things like the speakers, drives, etc can be shared to reduce costI can't see that, if it's got the same spec it'll massively steal sales from the unibody, you might buy it for the fancy case but businesses and schools likely won't. So that approach is pretty much out.
I also wouldn't expect quadcore, my reading is the available chips are way outside the TDP of chips previously used in the macbook pro. Not impossible mind, just can't see it happening immediately...