I made a quick mockup about how the message thing can look.
I thought about just having 2 pivots or panorama pages with a contact list like you have now select a contact and swype t the left to chat with this person swipe to the right click a diffrent contact now it loads the chat of that contact. So that you will always have just 2 pivot pages, otherwise you might get lost if you have multiple people chatting at the same time
I made a mockup of how your conversations can look in metro style. Idk if you like it. I made it quickly the icons etc. are but ugly but just to get an idea. All the images are just copy and pasted from skype and wp7 haha so it was just pretty simple and wont pass the marketplace (not with the skype header at least).
It very similar to what you already made with the contactlist and just a logical next step of your work. I could code the layout for it for you ,but I dont think its that hard to code this design or to get these images.
(Ow and yeah the credits in the sample text is towards you ofcourse, great job, great project man!)