The desktop running as your uid and some mechanism to mount external media that appears are likely not the same thing, but I'm not really clear on the details. Are you running hald? I would suggest looking at the xfce4 and hald documentation to understand what's supposed to be happening. It may be that you have to set sysctl -w vfs.generic.usermount=1 (or put in sysctl.conf) to allow non-root users to mount filesystems.
Some feedback... Yes, hald is running on this machine. I logged in as root to see if the error occurred then as well; it did.I also executed the sysctl command you suggested Greg, but it made no obvious difference.
Ray