I was using kernel 2.6.9-1.681_FC3, and I have configured the usb disks at /etc/fstab this way: /dev/sda1 /media/usbdisk vfat pamconsole,noatime,sync,exec,noauto 0 0 (I deleted the option "managed" because it used to give problems). I could plug, mount, unmount and unplug a usb disk, then connect it again to another USB port and mount it again. After updating to Kernel 2.6.9-1.724_FC3, it did not work: after unmounting and unpluging the USB drive, I cannot plug and mount it again, because the system considers that it is a new device sdb. Now, I am working again with kernel 2.6.9-1.681_FC3
Does the problem still happen with 2.6.10-1.727_FC3? It's a test kernel, available here: http://people.redhat.com/davej/kernels/Fedora/FC3/