iPad 1 stuck in DFU mode after 5.0.1 -> 5.1 OTA-update
This suggests that your NOR is failing/has failed. It could be a communications error also, but strange it gets so far before failing if it is a comms error.

Almost certainly this is hardware. If you still have extended warranty, use that. Otherwise you need to get this replaced via Apple's out of warranty program.
I'm not sure any third party hardware repair place could fix this, they would need to copy the unique data that is stored in another part of your NOR chip to a good NOR chip. This unique data is encrypted and I don't know if it can actually be copied successfully.

source: http://www.hackint0sh.org/f131/212842.htm