Its what Facetime and Qik should've been. Simply download Tango on your iPhone 4 or HTC EVO. Then dial your friends from your address book via Tango. Tango has no registration process and will tell you if your friend has Tango installed so you can call them. Video call over 3G data is decent and I had no problem doing so with other iPhone 4 or HTC EVO users.
Tango is an amazing app and I highly recommend it!