Ok, lets review the terms:
DVD-R (the standard ones for superdrives) - 4.7GB
DVD that you buy at the store (an actual holywood movie) - 8.5GB (already Dual layered
Dual Layered DVD-R - 8.5GB
So yes of coures your DVD player can play Dual layered DVD's it already has!
I am quite sure there are no notebook Dual Layered DVD burners out there yet but if you already have a single layered superdrive it is rumored that you might be able to get a firmware hack somewhere in the future to make it support dual layered. Also you need special Dual Layered discs to burn and those wont be out until later this summer I think and they will retail for $20. So dual layered is nice because you can hold a long movie but is way to expensive if you don't really need that much space.