Since my PowerBook died, I've been using a Vista powered Lenovo T60p, which isn't bad as far as PC laptops go. I do really miss the reliable sleep, overall sleek design, and ease of Wi-Fi handling that my deceased PowerBook had, though. I really don't need the optical drive except for the occasional ripping of a CD that I buy, or the even rarer DVD based software install. I'd be perfectly happy to leave the optical drive at home, only for use in those special cases.
The current line up of Mac Books is already very slim, it's hard to image how much more slim they'd get without the DVD drive baked in.
My main concern with the ultra-portable would be the limited size of the flash disk: I'd want enough space to stash media and a parallels vm.