Canon make a wifi grip for the 5D Mark II called the WFT-E4 II A, which advertises EOS Utility-based control of cameras over a network. This provides similar functionality with PC/Mac based Canon EOS Utility software. There are similar grips available for the Canon 1D range (1DmkIII onwards), 5DmkIII and 7D.

However the wording on the page you linked (and other similar Canon material) states, under "Compatible EOS Camera", that "The Canon EOS Remote app requires the use of a Canon EOS camera with built-in WiFi®"1. And then lists only the Canon 6D.

The language is pretty specific to the 6D and having integrated wifi, however given the app also hasn't been released yet (expected December 2012), so I'd guess not, but it might2 be a case of:

• A temporary marketing tool to drive interest in / focus on the brand new Canon 6D (it may well just work on any wifi-enabled EOS Utility-compatible camera, but for now they want to hype up the 6D).
• A permanent marketing tool to drive sales of new cameras, by artificially limiting compatibility to the 6D and future models only (in which case they might also release updated camera grips for current models, but that'll be the 5DmkIII, not the mkII).
• A technical upgrade to the network-based control protocol, without backwards compatibility. Perhaps they've had to update the protocol to support new features/functionality since network control was first introduced on the older wifi-grip-enabled cameras, and for some reason backwards compatibility isn't possible/worthwhile.

You probably won't get an answer until December.

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1. Not sure if the ® indicates some specific name of a Canon feature you need to look for in specs?
2. Warning: Pure spectulation!