Hi Paul,
You have a few options here. Several Mini users are using external DVD burners because they like the portability and they can buy one that's faster than what comes with the Mini. There are some DVD burners that also can be used both either by USB or Firewire. That's a nice option!
Another option you have is to buy one and try to install it yourself. I hear and read it's not that difficult if you don't mind voiding your Apple warranty.
Last, you can check out this company below. Here, you can send in your Mini for a Superdrive upgrade, hard drive upgrade, or memory upgrade, and it doesn't affect your Apple warranty! PLUS....their superdrive upgrade includes and 8x Superdrive rather than a 4x! Here's the link:
http://www.fastmac.com/ProductPage/minimax.html
Have a great day!