Yesterday, I reported that the Mac OS 10.5.6 update killed the iPhone's ability to get into DFU mode. Now there are two solutions: one is to connect the iPhone to the Mac via an USB hub. The other is to replace the IOUSBFamily.kext with the version from 10.5. Someone had created an AppleScript to do this and it is explained in the iPhoneWiki forum thread.
Original Store: WARNING: Mac OS 10.5.6 Breaks Hacked iPhone