Even though the Xbox 360 uses “standard” USB ports, not just any USB device will work with it. I don’t know all the legal specifics, but basically hardware must be certified and has some sort of “key” that will let it talk with the Xbox 360. If they didn’t do that, then you could use any old USB video camera, and M$ would lose out on major $$$
It appears that most USB keyboards are “unlocked”, so you can use them on the dashboard for typing an email, inputting character names, etc… but that’s the only “generic” USB device that currently works.
But as for using a standard Xbox controller on your 360, you’re out of luck. Some vendor may come up with an adapter, but I haven’t seen one yet.