Is it really the law in most places that you have to carry ID while riding a bicycle? Or is it just an unspoken rule that you'll get hassled if you don't?

Personally, I misplaced my driver's license 3 months ago and have been very relaxed about getting down to the BMV to replace it. (And I've even driven the family car during that time too - don't tell anyone! ) Probably I'll finally do it tomorrow morning.