In a controlled environment this would work, just like any number of other theoretical attacks. In the real world you have to consider people walking through the room, user shifting their computer or tilting the screen. You also have to account for other WiFi transmissions from other devices, and probably a number of other things I can't think off right now. Plus, it would require you to make keystroke maps for all kinds of laptops, since it wouldn't work in a universal way. Every laptop would have it's own pattern.

If you have enough control over the environment and enough information about the subject to make this effective, wouldn't it be easier to simply hide a camera in the room? That said, it's a really cool idea!!! I wonder what kind of other data could be pulled from a laptop by looking at it's EM emissions.