This seems to happen on a regular basis. WordPress simply stops listing new blog posts under category headings. No explanation, no nothing.
My new post, at http://loomisnews.wordpress.com/2008/07/20/republicans-losing-ground-in-senate/
is not listed at any of the 12 categories I've listed it under. (This is a new secret limit on category listings that is imposed by WordPress administrators without telling anyone about it until they complain, as I found out last time I turned to the forums for help -- at http://en.forums.wordpress.com/topic.php?id=31424&replies=9#post-229117
This has happened before. The only solution anyone mentioned was changing the language in "settings", logging out, then going back in and re-setting the language in English. (This "fix" no longer helps -- didn't help last time, doesn't help this time.)
I also tried re-editing the post, changing the number of categories, re-saving it. Didn't help.
This is actually one of the reasons I started blogging at WordPress, but it seems to stopped working A LOT ON A REGULAR BASIS.