I've just done a bit of googling around to try and find more on this. I've turned up a heap of references to the developers finding 6 different types of users during their 2 years of internal testing before launch. Alas no where is there actually a list of these 6 different types of users.

The doc in question was written by Kevin Fox who replied to this quora thread about it, but dodged the question claiming that the info isn't his to divulge.

http://www.quora.com/Gmail/What-are-the-six-types-of-email-u...

Weird.