For users that have just updated to WordPress 2.0.6 (you have updated, haven't you?), you might notice some problems with the feed and how it plays with The Feedburner service. Unfortunately, there's a bit of a problem that can cause it to serve up a 304 error when the feed's requested.
The WordPress folks know about it, though, and have already responded with what you'll need to fix (a small amount of PHP code) to get things back in working order.
It's actually related to the Server 500 error issue that 2.0.5 had (and 2.0.6 fixes)'¦ it just affects a different set of people. The solution to one problem caused the other. One symptom of the problem is the following error message from FeedBurner as it tries to request a WordPress 2.0.6 feed.
They've even isolated it down to those running mod_php on their servers as opposed to FastCGI/CGI or Lighttp users. Users experiencing the issue can grab this updated functions.php file or a patch if you so choose. There's even a guide to editing it manually if you'd like to get your hands dirty.