There is a current vulnerability with skype accounts where anyone who knows your email address can get access to your account, and even your private chat logs.
According to Kaspersky Labs (a leading computer security firm), the leader of the Russian Opposition has had his private chat logs leaked via this method.
This is a serious vulnerability and Microsoft (the current owners of skype) are looking into the issue. At present there’s no way to avoid the compromise, perhaps think about clearing your logs if you have anything private in there.
There’s a good article here that explains in more detail.