This got me a thinking, I am using CS3 so I don't even think I would be able to process a 5D mkII file with ACR should I be assigned such a file.
I'm not certain, but I suspect the current version of Adobe's dng converter (free download) is likely to be able to handle most of the current files, such as from a 5D MKII. After converting, you should be able to process the dng raw file in ACR, even with CS3, or perhaps even with CS2 (I forget how far back the dng format goes).
I've got LR 2.5 (and 3 Beta), CS4, DxO 5, Raw Shooter premium 2006 (!), Canon's DPP, and C1 LE version 3.77 available to cover most files people might throw at me. And then there is the dng converter, if necessary.