In and of itself, I'm all about a thinner (and lighter) 15 and 17 design. I just hope it doesn't come with too many compromises. As a Macbook "Pro" it will still need to be a powerful and capable computer. Can't be too skimpy on storage either, even if it has a fast SSD. I would want something with connectivity too, with enough USB and firewire ports, etc. Yes I know that in theory thunderbolt can give you those ports... but I still want real ports. I have yet to see thunderbolt to usb or firewire adapters... and while I'm sure they'll be out someday... someday is not today! Especially if these new laptops are really coming out before Christmas... which would be awesome...
Not sure how I feel about the lack of optical drive though. I do use them, sometimes, partly for installs, but mostly for music production related tasks. I don't mind, in an of itself, using an external superdrive... I just don't like having to pay extra for something like that. Especially not with the expensive price of macs... Maybe if they included one for free! (other companies have done that before, with ultraportable laptops).
But, like anything technologically related, we'll have to see what happens, and we'll have to see exactly what changes, improvements, and trade-offs have been made. I'm still generally excited at the possibilities...