Tommcd, like yourself, I too had discovered that it would work to allow for single-click inside Thunar, but that makes for a quite confusing desktop, double-clicking on Desktop icons and then single-clicking if they are in the file manager.
On my other laptop, that has regular Gnome-based Ubuntu 8.10, single-click works all the time, so I am guessing that it is NOT an Ubuntu problem, but rather one with the Xfce environment.
I also have an EeePC with Fluxflux-eee installed. This is a derivative of PCLinuxOS and it uses Openbox as the window manager. It too uses single-click, both on the desktop and in Rox, the file manager.
I like Xfce, but if I cannot find a solution to this, I will probably be forced to use a different desktop manager/environment.