You've unlocked your iPhone because you didn't want to deal with AT&T. Now you want to use FaceTime, but you aren't on a supported carrier.
MiTime will let your unlocked, jailbroken iPhone and is available through Cydia. Currently, it seems that sales are on hold until the new version of the App launches.
Since we do not have the software, we cannot test it out for ourselves, but the official website has the instructions. It advises users to "READ THESE INSTRUCTIONS very carefully. Everything below is included for a good reason."
The instructions are here, and while the process isn't as straightforward as FaceTime on an out-of-the-box iPhone, it should be very easy to anyone who has already been able to unlock their phone.