Hack the Planet weblog as a post about different service offering to save bandwidth for users and webmasters. Of the two listed on that post, I think that the most promising is RSScache.com. RSScache.com is a web proxy that supports rss feeds.
I noticed that there some latenency with RSScache.com. When I
created a RSScache feed to one of my rss feeds, and I forced my
aggregator to reload the feed, my web server only got hit once.
It looks like RSScache only hits the web server once every x
minutes. I am not sure what the frequency is.