Interesting, I was just talking about this with some colleagues.
I guess it just never occurred to me to do this - I've been brushing my teeth with tap water throughout the world for years in countries many would consider suspect (e.g., India, China, Thailand, Indonesia). Perhaps I am lucky, perhaps it is due to staying in Western quality (typically US based chain) hotels, but I have never gotten sick. I, of course, do not drink the tap water.
I'm actually more concerned about the tap water on planes and refuse to brush using that.