I recently attended a talk on English grammar given by Michael Swan and he pointed out some changes in modal usage. Firstly there are changes in the way some modals are being used, like the use of you may have been klled instead of you might have been killed when we know that the person didn't die. He also said that we are using modals less nowadays, an observation that reflects on quite an important change in language, one I hadn't noticed at all.
However, there is evidence that in one area of language use, computer-mediated communication (CMC), we use modals significantly more than in spoken or written language. I wonder whether the spontaneity of elecronic communcation, falling somewhere between written and spoken language makes modal verbs especially useful for expressing attitudes and relating to the recipient(s) of the communication we use.