Thanks to anybody for sharing your opinions and thoughts. I decided that the default rate limit functions will operate completely case-insensitive, because this seems to be what people (including me) expect. As most people also seem to have expected that behaviour in the past, I think this is one of the seldom cases where it makes sense to 'break' backwards compatibility.
Additionally I will add rate() functions with different names that treat the local part case-sensitive, so that there will be an option to control the behaviour per domain/address (or even other criteria) for those who need it (condolences). Again, thank you. Jan
