I used the instructions in http://thesalmonfarm.org/blog/2008/01/16/how-to-make-a-backup-image-of-an-usb-stick-or-sd-card/
to backup my SD card onto a USB stick of the same size. The backing-up was done onto a USB hard disc connected to the Sheeva plug; the backup file was then 'bunzipped2' onto the USB stick using a linux pc - bit convoluted as I cannot connect the USB hard disc and stick to the sheeva simultaneously.
Worked a treat - if /etc/fstab is written using UUIDs in place of /dev/sda or /dev/mmcblk0 then no changes are required - it will pick up the partitions perfectly. All that was required was a change to the u-boot environmental parameters to pick up the USB in place of the SD card
It should would just as well going the other way.