I have been having a problem where Nautilus launches when USB drives are inserted. I figured out how to disable this option in gconf-editor. I set volume_manager autobrowse to Off. I was looking at how I could change the autobrowse command to Thunar. I actually looked through the source code for volume-manager and found:
#define NAUTILUS_COMMAND "/usr/bin/nautilus -n --no-desktop %m"
Ugh, it is currently hard coded! It looks like a patch will have to be supplied to fix this problem for XFCE4 and other DEs.
Author of: MenuX Firefox extension, MonkeyMenu Seamonkey extension, SeaFox GTK theme, and Volume Desklet