I just did it for my 5D. Thanks for the mention.
But I would like to offer a warning to anyone else out there who, like me, hasn't previously updated camera firmware: RTFuM!!!, by which I mean: "Read The Firmware update Manual"!
So here is my own sad saga, which I share with the world in hopes that it may prevent some other poor sucker from making the same stupid, lamebrained, careless error that I did (Query about Forum Etiquette here: I hope that Ad Hominem attacks are permitted when they are inflicted only on oneself.)
So I "read" through the instructions (I'm usually a pretty careful reader, but . . .) and noted that it said (I thought) that you needed an 8 GB memory card to do the upgrade. I idly wondered why such a big card, but I went out and got a new Sandisk 8GB Extreme III which I'd been wanting anyway -- my biggest previous card being 4GB.
I downloaded the zipped firmware file, unzipped it, and copied it to my new card. No problemo. Meantime I had charged fully one of my batteries. I inserted fully-charged battery and 8GB card in camera and started the update procedure. But instead of showing me a progress screen, my camera screen went blank. Ouch! I waited a while, but it seemed that nothing was happening, so I turned it off.
Yes, there had been a warning that "Pressing any button or turning off the power while the upgrade is happening may disable the camera." But with no progress, what was I to do?
So then I reread the instructions, more carefully, and realized it said I needed an 8 MB card, not 8 GB (doh!!!) Actually, at least 8MB and "not greater than 2GB". So I got one of my 1 GB cards and loaded the firmware file onto it and tried again. And guess what? My camera was indeed disabled. Nothing would come up on the screen at all.
So I searched the web for anything about "Canon firmware update failure" and found lots of suggestions that "turning off the power may disable the camera", but no suggestions about how to do a fix. So it seemed that a call to the New Jersey Canon shop might be needed, but they close at 4:30 and it was now 4:29 and a half. So I'll call on Monday.
But then I noticed in one forum a suggestion that maybe taking out the battery for a bit and reinserting it might restore the old firmware. So I tried that, figuring that Murphy would guarantee that nothing that simple could possibly work. But, mirabile dictu it worked! My camera started right up, and I was able to install the new firmware (with the 1GB card and another freshly charged battery) just fine.
So there was a happy ending after all.
But if you are planning to try this at home, please, PLEASE, RTFuM!
Eric (who will never make that exact mistake again. But there are plenty of others . . . )