Steve Rachui is at it again. This time he's explaining how to setup R2's OSD multicasting feature. It's a great post and contains some very important information that you should be aware of if you're going to implement multicasting--definately a must read.
Steve's post is here: http://blogs.msdn.com/steverac/archive/2008/10/19/setting-up-multicasting-in-sccm.aspx
~Jeff