A web service (or maybe just a ASP page) renders and serves an XML generated based on the inputs passed to it.
A web browser picks it up, uses XSLT to transform it into a lovely page.

And what do you get, an application which does not waste too much time on the server.

Do not speak, just think.....Does nt your mind do a full revolution???

Anyway my friend really sparked that idea, said he has seen it implemented somewhere. But it was scrapped, cos of the sheer volume. (i dont wonder what he is talking about).
Just as an afterthought i do wonder how long will Gmail function. (wont it be complicated using XmlHTTPRequest all over your page).