If you disable WiFi and connect your phone using the USB cable and open Zune, your phone should be using the internet of the computer automatically. If you now use http://www.wireshark.org/ you can see the actual traffic.
Whether a secure connection is used depends from application to application, so one could go and create a community wiki answer to list them. In any case, this is something very interesting and it might be handy to bring this to the attention of the responsible developers that their are security issues.
Bottom line of the story: Make sure applications only can send/receive personal data when you access those applications, don't let them do background traffic. Together with that, don't access critical personal data in public. Other than that, consider a "privacy is dead" viewpoint, because unless your a very important / valuable person there's most likely not much interest in your data.
But, be wary of any traffic you do on WiFi networks involving hundreds of people...
Update: Yes, at least the People Hub's traffic is encrypted.