The WSJ's Lee Gomes talks to the heads of Gmail, Hotmail, and Yahoo! Mail. Much discussion of Ajax mail clients ensues. The Yahoo! guy (Ethan Diamond) was the Oddpost guy before they were bought out. He makes the point that their technology allows users to scroll through a big inbox very naturally -- Gmail and Hotmail still can't do that, relying on ugly "next/prev" buttons.
Similarly, the Scalix web client is the only enterprise product that does this -- Exchange's OWA can't, even in Exchange 12.
Read more here.