These words for the WordPress Guru, Mark Jaquith:
Despite two release candidates and several weeks of availability for testing, a bug sneaked into WordPress 2.0.6 that affects the serving of 304 Not Modified feeds. 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.
See Mark's post and fix: WordPress 2.0.6 Feedburner issue and fix
The fix is to comment-out lines 2231,2233,2234 in
wp-includes/functions.php. Please note that 2232 is left alone. The four lines 2231-2234 should look like this:
// if ( substr(php_sapi_name(), 0, 3) == 'cgi' )
@header("HTTP/1.1 $header $text");
// @header("Status: $header $text");