Here's the Privacy section from the Skype FAQ: http://www.skype.com/help_faq.html
Skype has been discussed a bit. The big problems are - It's a proprietary protocol they're not documenting, so there's no way to tell if it would be any good if they've implemented it properly, and - It's not an open-source implementation, so you can't inspect it to see if they _have_ implemented it well, and - The fact that they'd do either one of those things doesn't suggest that they're sensitive to cryptographic concerns, and therefore suggests that they're likely to have screwed up, and - They talk about end-to-end encryption but don't mention key exchange or user authentication, which says that at least their documentation and PR folks don't have a clue, and - The fact that they're using proprietary protocols says that they won't have an easy time finding commercial off-the-shelf equipment to build gateways to the public network, so they'll be less useful, and other people won't be able to develop cooperative products, unlike all the SIP and H.323 periphery that people are developing, but- other than that, it sounds like it could be an interesting system, and we could use some interesting user relationship models.