It is actually possible to fit a 8x DL SuperDrive into the MacBook, but Apple didn't, because it would be stupid putting such a thing in a $1099 machine (1.83 GHz White MB), and not in a $2499 machine (2.16 GHz 15" MBP). This is possible for 3rd party retailers to install, because the MacBook is 2.75 cm, while the 15" MacBook Pro is 2.59 cm. This is a difference of 1.6 mm, yet it is enough to make a DL SD possible.
It is not true though, that only Pro's have 8x DL SuperDrives. With a Core Duo Mini it comes as standard, and can for $50 be upgraded to 8x DL SD on the Core Solo Mini.All iMacs has it as standard too. It is only on the laptop area, where it is the 17" MBP with such a drive. I belive next gen. 15" MBP's will have a 8x DL SuperDrive, when Apple developed a SD thin enough. Then also MacBook's will have it a standard (I believe the low-end white one will just be upgraded to only have a 4x SuperDtive though. It has a Combo drive now).
Duallayer DVD's can come in handy if you wish to burn your movie collection on to as few DVD's as possible (singlelayer DVD's have 4.7 GB, while DL has 8.5 GB). If you want to take a backup of your HDD, an external HDD is much easier, faster and cheaper. More bytes pr. buck.
If you want an MBP with a DL drive, you don't have to buy the 17". Here it's much easier to get an external 16x DL burner, like the one in the PowerMac G5. But IMO, anything above 8x is useless is waste, because 16x DVD's are extremely overpriced. For me, 4x is fine, and I don't need DL capability, though it will be nice to see 8x DL SD as standard in all MBP's and high-end MB's in rev. B.
