I first saw them on Twitter as a flickr staff member had used one. I googled and found this blog entry on flickr short urls.
So this is just to spread the word. At the moment you have to view source on the photo page and look for:
<link rev="canonical" type="text/html" href="http://flic.kr/p/xxxxxx" >
But this then gives you a permanent short url that isn't going to expire and you also know where you are being taken to. I imagine when they officially announce it they'll add it into the Share This bit on each photo's page.