[ugh]

Hopefully the same as when you plug a Mac into another Mac via FireWire. (One of the Macs becomes the most expensive external HDD ever. This requires one Mac to reboot into the target disk mode. It would be nice if the same trick would be possible with Thunderbolt without rebooting.)

[GHFigs]

You don't have to use TDM. You can also use Firewire as a network interface.

[irons]

You have to boot into target disk mode for the same reason you can't plug one Firewire disk into two computers simultaneously.

TDM is supported in Thunderbolt, to my great glee. Terrific Firewire feature.