http://support.apple.com/kb/TS1275?locale=en_GB
It's just happened to me again while updating to 4.2. Every single time I have tried to update the OS on my iPhone it has been left bricked by the process breaking down midway and having to restore my iPhone from scratch - then go through the tedious process of restoring all my backups and media and then finally waiting for it to be reactivated by O2. I've tried using alternative computers, new leads, different USB ports, turning off all firewalls - nothing has worked except this:
but all it enables me to do is restore the phone - it doesn't stop the problem occurring in the first place. Anyone else had this and found a better solution?* While the iPhone is in DFU (black screen mode) go to task manager
(ctl+alt+del on your keyboard).
* in task manager go to the 'processes' tab
* look down the list and find itunes.exe and right click on it and
select 'end process'
* itunes should now close
* note iPhone should still be connected during this and have the
black screen still
* restart iTunes and it will recognise the iphone and ask to
restore the iPhone
* follow through by clicking on restore iPhone
* it will extract the software again yet this time the firmware
update should go through
to force into dfu mode
1 turn off
2 insert usb cable while holding home button