P2P was a dismal failure for both the BBC iPlayer and for Channel4's 4OD, being quietly dropped from both deployments; now it's making the Sky Player service unusable (unless anything has changed since I dropped that service a couple of months ago). You can find out more about why P2P 1.0 didn't work here.
But it's back, with another classic iptv moniker attached - multicasting. Now, multicasting - sending one stream to many viewers as opposed to one stream to each viewer - has issues of its own, the main one being that it requires IPv6 to function - and the internet world is still stuck with IPv4.
Adobe's new Flash Player - a radical re-working of the world's most popular browser plug-in - provides support for P2P multicasting. To those of us who live and breathe internet TV, this is actually quite exciting, especially as a server, rather than a client technology (I've long been a fan - and customer - of Velocix, recently acquired by Alcatel, who take a similar technological approach within their CDN network).
Here's a full presentation on the technology from Adobe's Matthew Kaufman: