I know what caused it, but I can't seem to find any info on how to fix it:
I fitted a skin to my iPhone - the type that you have to wet & slide it onto the phone then wrap it round. Anyway I followed the instructions & did it with the phone powered off, but as I was squeezing the water out the phone suddenly turned itself on. I thought nothing much of it, turned it off & continued fitting the skin. anyway, now when I turn on the phone I get the message "This accessory is not made to work with iphone" - even though nothing is plugged in, so I guess that water got into the dock connector & shorted something out. I've dried the phone out, blown compressed air into the connector etc etc, but nothing makes a difference. The phone still works, but only through the headphones & these have to be unplugged & plugged in again every time the phone is switched on.
Is there any way to make the iPhone work properly again, other than sending it in for repair/replacement (big pain in the ar*e, as I got it off ebay). I was hoping that someone may have made a hack for the firmware to prevent this error message occuring.