This is definitely a good start but we don't have this just yet without FireWire.Here's a article that claims it could be done with a special smart cable:
http://www.christopherprice.net/yes-usb-target-disk-mode-is-needed-badly-627.html
But if it eventually does come back, it might well be implemented with eSATA:
http://arstechnica.com/journals/apple.ars/2008/08/22/apple-aims-to-patent-target-disk-mode-for-esata