I just finished listening to "Brian Capouch – Asterisk Open-Source VoIP PBX" and it is very good.  Well worth the time invested.  I have to agree that Asterisk definitely has a serious buzz.  In summary:

Asterisk is the open source Voice-over-IP solution that everyone’s talking
about, and at OSCON Scott Mace talked to Brian Capouch, the author of the
forthcoming Addison-Wesley book about Asterisk. Capouch discusses why Asterisk is spreading like wildfire, has hooks for video and presence extensions and why Skype is "evil incarnate." [SOURCE: "IT Conversations News: October 10, 2005" newsletter]