It seems from my early work that there is one huge omission in the out-of-the-box WordPress install. It doesn’t do the Last-Modified header, on either the web pages or the RSS. I’m of the opinion that everything that emits or reads an RSS feed MUST use or honor this header. When you have a few thousand people subscribed to your feed, why not make it as simple as possible for those ones that check more frequently to know when not to download that 50K? That was one of my first additions to get_enclosures beyond the basic – storing a cache of when the last fetch of a feed was received and setting the If-Modified-Since header of the HTTP connection. This crap exists in the protocol, so why not use it?
I don’t know if there is a plugin that does this, but I know that if not that’s one of my first orders of business – to write one.
Update: Now I see. The themes I’m using are all using http://evilgeniuschronicles.org/?feed=rss2 as the link to the RSS feed, but if I use http://evilgeniuschronicles.org/wp-rss2.php then you do get the header set correctly. False alarm, citizens. Sorry about that.