what kind of HID bulb are we talking about here?
your average metal halide or sodium bulb is not going to like being PWM'd. They do make dimmable metal halide ballasts, and they require special bulbs which support dimming.
General Electric had a press release a few years back regarding a new ballast designed for industrial / commercial applications, designed to save money - a warehouse could have its lights dimmed during off-peak hours when minimal staff was present or a grocery store was closed, while still providing sufficient light for cleaning crews and security personnel.
If you're talking cold cathode fluorescent, then PWM is a good solution.