There are plenty of external usb dvd writers around ranging from about £55 from what I've seen. I use an internal drive (just bought a 16x, dual layer writer for <£25) - which is just about as robust but not quite so nice looking - connected via a usb-to-ide cable, or you can use an external usb enclosure to do the same thing more neatly (each about £20).
Using the ext enclosure route, you can upgrade the drive, or replace it when it breaks, on its own and you can use other ide peripherals in it such as internal HDD etc - I use an internal (again cheaper than external) HDD for backups.
USB2 makes a big difference here too. I've just upgraded a usb1 MB to usb2 with one of these
usb card/front bay - great 2 ports at the back, 4 on the front, plug and play and <£10.