Did you just update your Leopard system to 10.5.6?  Are you have DFU problems with your iPhone?

Lots of users have been experiencing problems with the use of DFU mode after applying yesterday’s 10.5.6 system update.

We believe this behavior is due to a kernel bug not a specific countermeasure by Apple. Possible fixes are (try at your own risk!) -

1. Replace the following plugin kexts from within IOUSBFamily.kext with the ones from 10.5.5 and then rebuild kextcache (if you don’t understand this, then you shouldn’t try it!)

/System/Library/Extensions/IOUSBFamily.kext/Contents/PlugIns/AppleUSBHub.kext

/System/Library/Extensions/IOUSBFamily.kext/Contents/PlugIns/IOUSBCompositeDriver.kext

2. Use a USB hub in between the DFU device and the Mac and insert/reinsert the iPhone’s USB cable.

3. Use a PwnageTool created .ipsw on Windows

-Thanks for the help Dev Team!