Source : ARS TechnicaApple registered a patent describing how to boot a Mac in Target Disk Mode via an eSATA port. For Now, one can do so using the firewire port.
Next step will probably be the addition of eSATA ports on all Macs. This may be helped by the fact that a new eSATA standard will allow to retrieve power from port.
On the other hand, it will be harsh competition for the Firewire, at least if you don't need to use multiple peripherals chained one behind another, or if you're looking for the fastest Data transfer.
[translation by Goulwen]
