...and what does that mean exactly?
So sites like the Guardian & the BBC, who were both quick to start publishing RSS feeds back in 2004-05, did this because it would increase repeat traffic to their sites: by broadcasting a headline to people who had expressed an interest in receiving such headlines, lots of said people would click through to find out more. Which is obviously a good strategy, and very similar to emailing them once a week, which all the press publishers also still do.
Publishing a taster on a 'click to find out more' basis is very different to offering the whole article for to be dragged into whatever reader you want though. A headline feed was only ever going to drag the reader back to the original site. A full feed can be consumed anywhere (personally as a Google Reader feed on the Viigo app on my mobile). As Mashable points out, it can also being to exist virally as Google Reader content is shared via Friendfeed.
I'd expect that this will offset whatever ad revenue is lost through onsite traffic, and the Guardian can continue to push their credentials as the most forward-thinking UK old media brand. However there is also a promise that
advertising will soon appear within each full content feed item
which will be interesting to follow as well...