As Anne Zelenka points out in her post "Busyness vs. Burst: Why Corporate Web Workers Look Unproductive" email has become a single channel of communication for the busy.
Blogs, wikis and RSS offer additional ways to communicate besides email.
Email was a relative easy transition from letters because the paradigm was the same. You write text and address it to someone. I still occasionally receive emails with the full letter text in the message (Dear Doug: . . . ). Email even carries over the antiquated "cc" and 'bcc" concepts from the days of carbon paper to produce copies. (Carbon paper for letters disappeared with typewriters).
Email was cheaper and faster than conventional letters, so it is easy to see why its use became so widespread.
Unfortunately, email has quickly become the only communication tool, rather than one of the communication tools. I often will get stuck in an email thread could have been better dealt with on phone call.
The popularity of the blackberry has solidified the prominence of email as the primary communication tool. Being freed from the shackles of you ethernet cable, all of your email can be hanging on your belt.
Email is the knee-jerk response for communication. Everything can go in there: correspondence, contacts, reminders, documents, to-do lists, etc.
If you look at your email traffic you may realize that all of that email need not be in your inbox. Much of it you do not need to respond immediately (if ever).
A blog can be a better tool if you are announcing something. A wiki can be a better tool for archiving information. Both of these are better ways of sharing information and are more retrievable across the enterprise than email. RSS alerts can be used to promulgate this information through less disruptive means than email.