Percentages Of E-mail Clients By OS And By Feature?

Krotus asks: "I've been looking for information on user shares of popular e-mail clients, and am really surprised at how hard it is to find anything beyond vague speculations. I suspect that lots of us could use these numbers, but has anyone been able to find out about what percentages of users are using Eudora, Outlook's various versions, mutt, pine, mh, Notes, etc.? Almost as useful would be numbers on what percentage of clients can parse HTML, vs. plaintext or something proprietary. Forrester, Jupiter, and IDC have all come up blank. The question has been asked at Abuzz with no luck. Maybe the collective knowledge of Slashdot will be more fruitful?"