My understanding is that with the redesigned 5G iPods, they lost the firewire bus, which is what allowed the ipod to be recognized as a startup drive. Can't use a USB drive to start up in OSX, and the iPods now are strictly USB. You can power any ipod in every generation with firewire, but can't transmit data to any ipod over firewire. The shuffle, nano, and video ipod 5Gs, and every subsequent ipod, are all USB only.
Here's a recent thread discussion on this topic. When the 5G ipods were released, there were numerous threads about the loss of the Firewire data channels and the effect that had [including the inability to use it as a bootable drive.]
Note that the thread I linked also begins to discuss why you shouldn't use the ipod as an external drive for heavy use [including simply booting]: it has no ability to dissipate heat at all, and will constantly spin, generating lots of heat, which leads to the breakdown of the hardware.
I can't use my 5G iPod as a startup, but I keep my 3G around for just that purpose in the case of emergency.