An Ubuntu Forum staff member (aysiu) has just posted a HowTo on the Ubuntu Forums on using a few scripts that he wrote (with the help of ciscosurfer) to change the default file manager in Gnome to Konqueror or Thunar. He also provides a script to revert back to Nautilus if you aren’t happy.
Since I’m currently using Krusader as my file manager, I’m hoping he’ll also include a script to change the default to it (I’m not good at scripting, yet!).
You can find his instructions and scripts on his website. (Great resource, by the way!)