I finally figured out why I couldn't get XBMC to stream Frontline shows anymore, it turns out PBS has changed the URL of their media server from a static IP to a real hostname. The script was originally written to use the static IP, so I fixed it. I also noticed they changed the URLs slightly so I allowed the script to handle that as well.
If you have XBMC running on your Xbox, make sure you're running a relatively recent install. (The one on Slayer's won't cut it.) You can grab the script and extract it in your XBMC/scripts directory. Once you run the script it will grab the listing of all of the Frontline episodes available on the web along with a description of that episode. (This isn't to say if you don't have an Xbox you can't watch Frontline, the videos are made to stream to your PC, it's just more comfy to sit on the couch and watch them!)
Update: I've submitted my updated/fixed version to xbmcscripts, and it has been posted. You can download it here, or just grab it from my files sections here: pbs-v.0.2c.zip.