FaceTime Missing From iPhone 4 After Restore (and Fix)
This is an issue I came across after restoring my iPhone 4 from a backed up 3GS running iOS4 Gold Master. I don’t know if It has to do with GM or anyone who is restoring from a previous device running iOS4 will some how overwrite the FaceTime feature to the extent where it’s missing from the phone app and also excluded from the settings where normally you would enable or disable it. I have found that restoring your and setting it up as a new Phone corrects this issue. I am currently working on that will update if it works!
UPDATE: So I have found that restoring your iPhone 4 and setting up as a new phone corrects this issue. FaceTime is now available in settings and in the phone app. Just remember that if you chose to do this you will lose all your custom phone settings and app specific data. If you have your phone syncing to an Address book, picture folders and calendars all your contacts , photos and events with be synced.

Hey I had the same problem – but found the fix here: http://www.ismashphone.com/2010/06/eye-see-nothing-the-case-of-the-missing-facetime.html
Looks like it is an option in settings under the “phone” category – just turn it on there.
Actually, this problem has to do with restoring and setting up from back up or even possibly restoring and setting up from backed up iOS 4.0 Gold Master. The option in settings to turn facetime on disappears complete and is only fixed with a restore and and setting the phone up as a new phone.