From reading some lighting forums, apparently it's illegal in some places to run an HID bulb in a light that wasn't designed for it. I know some of the "tuners" around here have annoyingly bright lights, like they have their high beams on all the time. Great for them seeing, but blinding to everyone they're coming towards.
Of course, it's like a lot of things that are "illegal", they have to feel like stopping you and checking on it, or maybe need another reason to stop you first, so enforcement is spotty at best.