For anyone who serves up content that is requested repeatedly by the same user agents (be they web browsers, news readers, or any other) it can quickly get expensive (financially and in performance hits) to use up bandwidth sending the same data to the same destination over and over. HTTP provides mechanisms to deal with that, but until recently they’ve been poorly supported.
Those costs are the reason that this is great news. DHH just committed a change to Edge Rails that automatically adds an ETag when sending a response and returns an HTTP 304 (Not Modified) when an agent comes back asking for the same content before it has changed.
Chad Fowler has a good explanation on his blog, where I heard the news, and this page from the Universal Feed Parser docs is also a good summary.