Later Jon goes on to ask:
So is Gmail a rich Internet application? Sure. Although that label most often applies to Java, .Net, and Flash clients, Gmail shows that Web clients can join the club too. But crucially, Gmail's architecture is open to other kinds of rich clients, too. It doesn't have to be a zero-sum game.
It's interesting that Gmail is manuevering between the categories of thick and thin clients. Maybe there should be a whole new category for 'slim' clients and Gmail and Oddpost are just the first entries.