I had a different problem: I had put the firmware file on the CF-card, but had a SD-card in the camera as well. As soon as I chose update from the menu, before even being asked which file to use, the camera appeard to freeze. After taking the battery and cards out, it eventually returned to normal state and I could continue using it, but, of course, I had to try again. In about the 5th try, I found the solution:
The camera seemed to be looking for the firmware on my SD-card. When I tried upgrading from the CF-card, without the SD-card present, the upgrade procedure ran straight through as expected, no problems!