Hey Tom, I second that request, although I wonder whether the browser is the best place to put this. I'd be tempted to put something nearer the connection itself (a proxy of sorts) that can traffic shape.
For instance, I find it worrying at times that Firefox and many other programs auto-check for updates and in some cases automatically download new versions. Although this is terribly irritating (and costly) on mobile internet, I am happy for it to go on while I'm at home.
In this case, I would expect the magical filtering program to prevent, throttle, or otherwise warn about these goings on.
Such a program could (I presume) act as a proxy to block unnecessary junk to save on precious bytes (only show images when prompted; block flash, etc)
Perhaps with a server-side companion it could also heavily compress images, HTML and other media before sending it over the pipe - a la Google Web Accelerator; Opera Mini (I think) and the server-side compressing done by Vodafone, etc.
- Sven