The Apple support rep I talked with, emailed me a list of FaceTime tips that were quite useful for troubleshooting and for usage. I’ve quoted them here for your benefit.

Troubleshooting steps to ensure you can connect to another iPhone 4 FaceTime user:

1. Make sure they are on a valid WiFi connection and can open up a webpage in Safari, of course.
2. Make sure FaceTime is activated by going into settings —> phone, and FaceTime is toggled on with this message displayed underneath: “Your phone number will be shared with people you call with FaceTime.” If this is not the case, and it says “Waiting for Activation,” confirm that they are able to send and receive text messages. This could indicate an issue we would need to resolve with AT&T.
3. It is required that your Caller ID is shown when initiating FaceTime. You can make sure this is on by going to settings —> phone, then “Show My Caller ID.” Make sure this is turned on.

Some useful tips for when you’re using FaceTime. Some are obvious, and others are “whoa, thats cool” stuff:

If all of these things are set up properly, you should be able to initiate FaceTime with no issues. Some tips on how it works:
1. Your preview panel, which shows what the person on the other line is seeing, can be moved to any of the 4 corners by clicking and dragging.
2. If you want the person you are speaking with to see the image from the rear facing camera, you can tap on the icon in the bottom right corner. Tap it again to change back to the front facing camera.
3. You can put the iPhone into portrait and landscape mode to change the viewing orientation for the person you are speaking with.
4. Just like a phone call, you can multitask while in a FaceTime conversation. You will be able to hear each other, but your image will be paused on their screen while you are accessing email, Safari, etc.
5. FaceTime uses no AT&T minutes! You can initiate a FaceTime conversation without making a phone call by tapping on the contact you want to perform a FaceTime chat with, then selecting the FaceTime option.

I didn’t know I could multitask. Perfect. Need to look up something while on FaceTime, no problem. :D I also like that I can move my preview panel around – just like in iChat.

If you have any other tips re: FaceTime, feel free to post them. This is truly game changing stuff we’re seeing at work here.