As far as I can tell.... IceShare is better than Peercast for a number of reasons.
Firstly, IceShare doesn't depend on a central server. Peercast depends on yp.peercast.org in order to function properly. Yes this is true, and recently peercast has become organised this way. Which i dislike.
Secondly, Peercast requires that you run the peercast client on each machine. If IceShare will be a library then lots of media players will be able to simply "play" a stream. People will just need a player that can do icet://
Thirdly, IceShare is based on ogg. If you look at yp.peercast.org you will find lots of formats. Most of the streams are not using open codecs. For example the video streams, there is just ONE theora stream, while the rest are WMV/NSV.