BTW, another curious part of firewire (unrelated to target mode) is that a firewire device can read and write RAM from a running PC, without interaction. Even when you have "locked the workstation". Random google link: http://www.hermann-uwe.de/blog/physical-memory-attacks-via-f...
Yes, a firewire has the ability to do whatever DMA requests it wants. This is a thruput advantage (especially when processors were slower) since the host CPU only has to set up the data transfer and the rest can happen in hardware. Back in the day, FW400 would beat USB2 in most benchmarks even though the raw bandwidth of USB2 is 20% higher.
The solution to this is to use an IOMMU, which protects memory from DMA traffic just like the CPU's MMU protects it from userland processes. However, I don't know if any current Mac laptops do this.
Thunderbolt, ExpressCard, and PCMCIA ports have the same issue although it'd require some fancier hardware to exploit. I think SD cards as well, but I'm not 100% sure about that.
USB isn't vulnerable to this because the protocol is more like a network card: devices send you packets rather than initiating direct DMA.
