Are there any special regulatory requirements for media storage in an email archiving system?

It depends on what industry and what regulations affect your email and archive. In reality, the only special media requirements that are imposed by the US government (and by any state governments) are on securities brokers/traders that do business in the United States -- their correspondence and their records must be stored on a WORM (write once read many) device such as a CD or DVD recorder to prevent data from being altered...