I'll be getting a MacBook soon, but I'm gonna fork out that bit more just for the extra processor speed and the Superdrive - only because I'd prefer to have my DVD RW be portable along with my mac (and I don't have any DVD-RW be it external or internal).
I'm pretty sure any usb DVD-RW should work with OSX Tiger BUT as Echo pointed out, you need the program Toast or the likes.
I would rather be abit more patient, save till I can afford a MacBook with the superdrive - add to re-sale-ability AND for all you know, the Rumour about a new processor for the MacBook maybe true and you'll be glad you waited a wee bit more?