Backup. If you can't do the job sufficiently with the level of quality required by the job then it's not backup. If you've promised clients 21 megapixels worth of resolution then 12 is not sufficient as backup. If you promise to shoot natural light in dark rooms then a 4/3 camera is not backup. If you're style is shallow DOF then an f4-5.6 lens is not backup. If you promise digital then film is not backup. Professional backup is not having something that will get the job done, it's having something that will get the job done just as well as your main camera/lens/whatever. There is no 'minimum necessity', if you can't do the job, to the same degree of efficiency, with your backup system then your backup is not sufficient, period.
Of course a 5D might be just as sufficient for your commercial usage as a mkII (in which case why are you buying the mkII? Business is not a case of latest toys, just buy a 2nd 5D) if so then it is a good backup to a mkII. If you promise higher res files or sell your services based on an understanding of such, it is no longer sufficient backup. Don't forget of course backup for lenses, cable releases, CF cards and all lighting that you might be using.
I'm a wedding photographer, I've learnt all of the above through the school of hard knocks (to cameras), I have a 3rd camera and flash because yes I've needed 3rd backup. I have a speed dial of rental shops who can courier me equipment mid wedding and I've had to do it before (not to use, just as backup, when your main gear dies and you resort to backup, what happens if murphy strikes again? you're already using the backup!).
When you need backup you need it for real. In the professional world it is very much for real.
