First off, yeah, this would be an innovation with gaming in mind at first. Now, arcades and home consoles have implemented pressure sensitive buttons for years, if not decades in the case of arcades. The add an entirely new dimension, feel and possibilities to the experience. Why haven't keyboards done this yet? The technology exists, and all it would really need to happen is a custom board with the switches laid out on top as normal. If anything about modern keyboards could be improved, I think this would one of them. Now PC devs haven't gone this rout because the technology doesn't exist in modern keyboards, and believe me when I say that the thought has crossed many a studio, but in the end it's not up to them , or more aptly, it's not in the budget to create custom peripherals of this nature.
Not only could it change gaming for the PC, but it could also change the way people type and use keyboards today. Think about the possibilities: custom settings for tapping, or holding a button on the keyboard. Layouts specifically tailored for your actuation force. The ability to switch between normal traditional typing and a pressure sensitive layout. Scrolling or moving at speeds that are determined by you and not preset. Imaging even being able to switch between tools in Photoshop, all bajillion of them, because each key has two different settings.