Does Facetime work over a cellular connection as well as Wifi
No, you need to jailbreak your device to enable Facetime over 3G as of iOS 5.

Here's an extract from the official documentation:


  With FaceTime, you can participate in video calls with another iPhone 4 or later, iPad 2, iPod touch (4th generation), or a FaceTime for Mac user over a Wi-Fi network connection.