Apart from the floppy disk drive, I have only tried one other USB device:
a Kodak DC 280 Digital Camera.
Although this is recognised as a device on the bus, if you try to mount it using (for example)
$ mount -t vfat /dev/sda1 /mnt/camera
you get a message telling you "/dev/sda1 is not a valid block device".
I believe there is something non-standard about the way the mass storage device is presented.
The camera does, however, work perfectly with gphoto. Goto Configure> Select Port/Camera Model and select the camera from the drop-down menu. Then under port, select "other" and enter /dev/usb/dc2xx0
To avoid problems with non-root user permissions, change them as follows:
$ chmod 666 /dev/usb/dc2xx0
Details of the support status of other USB devices under Linux can be found at:
http://www.qbik.ch/usb/devices/