This was the same with the PB's. Here's a long explanation. The 15" MBP's suderdrive is crossing the trackpad/keyboard, and needs to be placed lower to make room for the drive. This was the same with the 15" PB, but because it was 1.1" instead of the MBP's 1", there was room for a 8x DL burner. The next revision 15" MBP's will probably have a 8x DL burner, since they will have made thinner superdrive by then. The 17" MBP is the excactly same as the 17" PB, except the iSight and different ports.
Concerning the MacBook, which has a 4x non-DL burner. Due to it's thickness of 1.08", there is room for a 8x DL burner, but it would make no sense putting a 8x DL drive into a $1099 machine, and not in a $2499 machine.