Not to fuel a rumour, but this article seems to suggest that there is a backdoor in Skype that allows conversations to be easily tapped. While I think it is not necessarily true, I cannot but be concerned if, as the article says, Skype did not vehemently deny it.
I think Skype has a good thing going for it, and, frankly, the only way I will use it is if their protocol is published. An unpublished one is totally untrustworthy no matter how good.
But who needs skype anyway when we can do VOIP through a fully supported, standards based, open source solution that you can now find at http://talk.fedoraproject.org/?