Here's how I did it:
1.) Get your USB system running:
a.) Edit /etc/rc.d/rc.modules. Uncomment-out the line with reference to usb-uhci (for Intel and VIA chips)
b.) Add the following line to your /etc/rc.d/rc.local file:
mount -t usbdevfs none /proc/bus/usb
2.) Plug-in your Rio600, turn it on, reboot.
3.) Grab yourself...er, I mean, get yourself a copy of "rioutil" from Sourceforge:
4.) Untar the file, cd to the newly created directory, then (as root):
$ ./configure --with-usbdevfs
$ make install
5.) As root (per my experience, rioutil won't work as user):
$ rioutil -i
You should see the RAM info for your Rio600. If so, then congratulations!
To upload songs into your Rio600:
$ rioutil -a /your-mp3-dir/song.mp3
To upload multiple files:
$ rioutil -b /your-mp3-dir/*.mp3
The rest of the commands can be found in the rioutil README.