I think it was quite clear from the day we were able to Facetime in action but it seems that many people are still wondering if this new feature of the iPhone 4 will use minutes of our data plans. The answer is resounding: no.
Facetime works under WI-FI connection so all data traffic is handled by the line to which we connect wirelessly. This means that both our voice minutes and data MB will remain intact in our rate.
Only when the jailbreak allows us to use Facetime under the 3G network (which will not take long to be done) will we really be able to tremble due to data consumption since it is known that image quality, low latency and high bandwidth are the prerequisites for Facetime.
Source: AppleInsider
In theory, with the cydia application "3G unrestrictor" it could already be called via 3G.
Does this mean that facetime can only be used between 4 iPhones?
At the moment yes. Apple is interested in Facetime becoming an open standard, but today it can only be used among iPhone 4 users.