The first video calling app for Windows Phone 7.5 Mango devices is now available. Tango has been available on other platforms and is finally available for Windows Phone 7 devices. Check out the Windows Phone Marketplace for the download.
Tango does not require a front-facing camera and you can use the rear camera for video sharing. Obviously the app works better with a front facing camera, but it's nice to know you can use it without.