‘According to some iPhone news sites, the latest Mac OS X release (10.5.6) disables the use of DFU mode. This can be bypassed by plugging your iPhone into a USB hub. The Dev Team has been notified on this and a fix will probably be here soon. For now, do not update to 10.5.6 if you want to jailbreak your device unless you absolutely have to…or have a spare USB hub.
Basically it scattered reports are claiming that everything will work properly if you are connected to a USB hub instead of directly to the computer. The idea behind this is that the drivers for the USB hub will still allow you to connect in DFU and do your thing where the updated drivers in 10.5.6 when you connect directly to the computer would be fubared. What also would probably work is if you took the 10.5.5 drivers and stuck em on 10.5.6.’