I have had a hard time finding decent overviews on OpenID, the specs are there (and they aren't too bad) but they are hardly a quick read. The linked presentation goes someway to addressing this.

BTW: The whole presentation is implemented in a single HTML page, controlled by CSS. Neat.

A 15 minute slideshow and podcast on VOIP security from O'Reilly's 2007 Emerging Telephony conference.

Back in 2005 Anagram Laboratories conducted what appears to be a complete security audit of Skype (v1.5 I think).  It is very positive, although personnaly I would have liked more technical details published.