Dear un-scientific fellas:
1.Ask and define the question.
2.Gather information and resources through observation.
3.Form a hypothesis.
4.Perform one or more experiments and collect and sort data.
5.Analyze the data.
6.Interpret the data and make conclusions that point to a hypothesis.
7.Formulate a "final" or "finished" hypothesis.
Result: Yes, FaceTime works on WiFi only.
TEST, TEST, TEST!!! Don't just believe info posted on the web or sarcastic remarks by some dude cough* douche! cough*.
I have successfully made several FaceTime calls using ONLY WiFi; by entering your phone in 'airplane mode' you disconnect it from the cellular network and keep the wifi only.
Then you go to contacts, choose the victim, and select 'facetime' and... Voilá!! You are calling without cellular network whatsoever. Of course the other party has to have iPhone 4 too because these expensive toys are the only ones with the technology. So far.
But that's not all, two of the calls I made this way were made INTERNATIONALLY. (Ha! Take that ridiculously expensive service providers!!!)
Also, WiFi is not really necessary to use use FaceTime, you could try 'freeing' your phone you know... wink* wink*
Any questions?