If you thought that VoIP calls over the internet were anonymous, think again. According to a recently published study, Tracking Anonymous Peer-to-Peer VoIP Calls on the Internet is possible by using a watermarking engine that changes the otherwise non-distinctive VoIP flow into a trackable unique stream by use of inter packet timing differences. I suggest you read the complete paper (PDF) to know all the nitty gritty details. If you have a hard time obtaining the paper, or the connection times out, use Google's cache.
Will this forever change how people use VoIP? I really doubt so, because even if less than 0.1% of the current VoIP users worldwide read this paper, it would be a huge success. Besides, reading the paper will only alert you to the possibility, but it won't fix or make your calls "untraceable". As long as third parties are able to come between you and the one you're calling, the VoIP stream can be watermarked and traced.
If you now think "but Skype encrypts all calls using 256 bit AES" you are correct, but missing the point. Encryption doesn't matter for this type of watermarking, as no one is trying to figure out what is being said, or what data is being transferred. This technique just proves computer A connected/talked to computer B at a certain time. Adding more encryption, or using an anonymizer network won't help.
I promise to post something "lighter to digest" over the weekend :)