I just ran a cool little utility called Comstats. It sniffs through my Mail.app mailboxes, and runs some stats on the contents. It generated this cool and somewhat scary graph of number of emails received per day, over the past 2 years:
It's a little scary to extrapolate what appears to be something loosely resembling an exponential curve out another couple of years. Yikes.
There's also a couple of other cool stats to run, including "Number of Emoticons Per Message" and "Number of Links Per Message".