The "320 and up" idea is interesting, never thought about building a site for mobile users then scaling it for the desktop users. It seems to me to be the same idea though, of scaling down a desktop site to a mobile site.
I have just started doing the responsive design thing with
@media queries, but I'm not too far into it and I'm starting to resize the pitfalls and the gothas with this method just might outweigh the pros.
I am looking into jQuery mobile, looks very interesting and the sites that are using it seem to be lickety-split performance wise. I also like the idea of using a jQuery since that is what I know. Takes more work, but with usability in mind, it seems to be the better route.
Addressing the bandwidth issue, Smashing Magazine in an article on "How to build a mobile site" tells us to set any unnecessary content and background images to
display: none;. The problem with this is, the images and content are still getting downloaded, just hidden on the client-side.