Thanks for the guide - I tried it using last nights build - magiclantern-v2.3.NEXT.2013Mar30.60D.550D.600D.50D.500D.5D2.1100D, sadly it didn't work, in fact the only thing it did was disable liveview.
I will confess to not doing the last step to ensure it autoboot, which I'm glad about actually.
It may have been just that build, but I did note the lack of 5D3 in the title.
The card was a previously working boot enabled card and I could load the firmware okay.
I'm not looking for help, I'm just reporting back what happened to me - thanks again for the guide though, no doubt it will be of use.
EDIT - I tried a couple more times and saw a message saying enableboot then a :-) followed by frozen or in one case an error 80.
I would suggest we hold off playing for now, or at the very least try a different build.
Even on the occasions where the firmware loaded and didn't lock it the liveview was disabled and the delete icon did nothing so I never saw a menu to try anything else.