I was sent a link by herb to a story on Wired News: Dave Winer comments that if implemented properly, the check for new content is an "infinitesimal" burden. That is, using the appropriate HTTP 1.1 headers.
Still, Winer said he would love to work on a solution system when this turns out to be a real problem. He goes on about having a group of aggregator developers to talk about these issues. Well, my gutt feeling is that polling the original content provider every half an hour is inefficient by any standard.
On the other hand, Tim Bray talks about
I believe that whether you are going to use Jabber pubsub like I said earlier, or some other way of spreading the load, something should be done. Jabber enabled desktop aggregators will probably not be in wide spread use any time soon, but what about feeding RSS mirrors using pubsub?