It is called immedi.at.
He described immedi.at as RSS via Instant Messaging service. At first I thought he was referring to a variation on something like MSN Alerts. MSN Alerts allows anyone with an RSS feed to register provide an alerts service. This allows blog / site visitors to sign-up for update alerts via MSN Alerts or email.
I was wrong.
What Peter was showing me was something quite different. What he showed me was a way for the blog / site visitor to subscribe to an RSS feed and be IM'd, without the blog / site owner having to do anything other than provide an RSS feed.
So to summarise: Peter's sweet service allows anyone to subscribe to any feed. His service pings the RSS feed for any changes and if your IM client is running, his service IMs you. Very, very cool indeed.
I'm also using this post as a test. The idea is that shortly after I hit 'post' my IM client should wake up and let me know. Let's see...
Update: It worked! I created this screencast to show how to use...