I read they are using a smaller/tinnier superdrive in the MacBook Pros that really doesn't support dual-layer DVD burning. By the way, check out the thread on the success of dual-layer media. After that you won't feel so sorry for no dual-layer burning support.
The express card slot could be used for additional USB slots, firewire 800, memory stick drives, upcoming wifi cards...
iBook 600; 12''; 640mb; 8mb Rage; DVD-CDRW-Combo, 20GB
P4 1.6; 2x80GB Raid1 (file-server)
tiBook 1Ghz, Superdrive, 768MB, 64mb 9000, 60GB