Any external USB or Firewire drive should work, but ideally you need to get the drive recognised by your Mac so that the iLife apps and the Finder work correctly with it, otherwise you will only be able to use it with a third-party app like Roxio Toast.
I bought a LaCie DVD±RW with LightScribe, Design by F.A. Porsche. Both the USB 2 and Firewire versions work fine with the Mac. Be aware that LaCie only supply Mac software with the more expensive Firewire version, although you can download the Mac software from the LaCie site. The only bit of software you won't be able to get if you buy the USB drive is the actual LightScribe labelling software - this only comes with the Firewire version.
There is a third-party LightScribe capable labelling app for the Mac - it is called Disclabel. The extra cost of this on top of the USB drive might tip the balance in favour of the Firewire version.
I bought the USB 2 LaCie drive and can confirm that once the Mac software is installed it works perfectly with my Mac, just the same as the internal DVD drive.
