I'm helping a friend set up a self-hosted Wordpress blog + feedburner and I'm seeing a problem with the feeds that I'm finding somewhat mysterious. Using the default permalink structure (e.g., ?p=123) everything works as expected; I can follow the feed in Google reader, navigate to it manually, and set it up in feedburner.

However, once I switch away from the default permalink structure, feedburner and feedvalidator both report that accessing the feed is returning HTTP-404 and Google reader no longer shows new posts (I'm assuming for the same reason), but I can navigate to the feed using a browser. When I do that it appears as though nothing is wrong; there is a feed there and it contains all the posts I expect it to have. I've re-started the feedburner & reader set-up from the beginning after changing the link structure, so I don't think they're doing anything silly like looking at the feed at its old address.

I've seen people with similar problems in various other places but there doesn't seem to be a good answer anywhere.