This problem just happened to me not too long ago. It was one of my awesome readers who alerted me. However, when I checked the feed using only my browser it showed up normal, but when I checked via Google Reader only then was I able to see the hacked feed. So people should be advised that it may show up normal, but they should check it through a service to be sure.

Just wanted to post a follow-up as I've officially tracked down some details about the problem (I'll also post to the Google page to let others know too).

To find out if you're affected, WP users should check their wp-content/plugins/ folders for any plugin files that begin with a period (i.e. .akismet.bak.php, etc.) and immediately delete them. It's important to check every plugin/plugin folder for these files as I found two on the same site lurking in different plugin folders.

Next, check the wp_options table for something like rss_f541b... and check for a string of code that looks like ';))"==QfK0wOpcyMyEzboNW...(edoced_46esab(lave' (the ... just signifies a bunch of additional gibberish characters) and immediately delete that chunk and update the table.

Once all this is done, it's super important to change your DB user/password along with your WP login details just to be on the safe side.

I just finished decrypting the entire file/DB code (which is just PHP code hiding in the DB) and it's nasty.

The above worked great but even after I changed the db un/pw and wordpress un/pw it showed up again. I can't seem to find any odd files so the only thing I can think of is to backup my posts, delete everything, and start over.