I have a Macbook Air with Lion. "Most of the time" / "Intermittently", when I plug in a USB device, the built-in keyboard/pad stop working. I find this fairly frustrating.
Does anyone know what's wrong?
Interesting situation, I'd try booting to Safe Mode (you can follow the relevant instructions here)
It sounds like there's an issue with your kernel extension caches, which booting to Safe Mode will clear, along with generating new kernel extension caches.
Let us know how you go!