FaceTime is an open protocol: http://www.readwriteweb.com/archives/apples_biggest_news_vid...
and
http://blog.imtc.org/index.php/2010/06/09/the-technology-beh...
and
http://blog.imtc.org/index.php/2010/06/09/the-technology-beh...
Where's the spec?
http://stackoverflow.com/questions/3115535/cant-find-ios-fac...
h.264: http://www.itu.int/rec/T-REC-H.264-200305-S
aac: http://www.arm.com/files/pdf/PRD10-GENC-001288-4-0.pdf
sip: http://tools.ietf.org/html/rfc3261
turn: http://tools.ietf.org/html/rfc5766
It may be open, but that doesn't guarantee adoption...to date, only the iPhone 4 makes use of it, whereas Fring and Qik already operate on multiple platforms. If Facetime does manage to catch on and other manufacturers adopt it, then the iPhone loses that competitive advantage, since there will be plenty of Android, Symbian, and Windows phones that use it. The other OSes have the advantage of being able to use other standards as well (because Apple will not allow an app that competes with Facetime in its app store).
Additionally, when someone buys an iPhone, they are not going to use that phone for the rest of their life. People who already have iPhones will buy new models. Others will migrate from iPhone to other phones. So just based on the number of phones sold, you can't tell when it will "catch up".
Furthermore, as Skype runs on the iPhone (over 3G as well now), iPhone users can also be Skype users.
You're right that you can't tell when the iPhone will catch up purely based on those numbers. I'd say it's a smart bet, though, that it won't be long. We know that Apple has already sold a few million iPhone 4s, and I'll bet that it sells somewhere in the neighborhood of 50 million before the next iPhone comes out.
You're also right that you can Skype on the iPhone. We don't yet know how good Skype video chat will be on the iPhone, how it will compare to FaceTime, or how many people will use it instead of FaceTime. We also don't know who else (if anyone) will use the FaceTime protocol.
However, we do know that every iPhone 4 comes with FaceTime enabled, that it is trivial to use, and that it is integrated in the phone and contacts at the system level. All of this softens the significance of Skype's apparent head start.
On the other hand, it makes FaceTime more useful, because you'll be able to call more people with your iPhone. Most likely the "advantage" will come down to who has the best implementation, and who has clunky implementations.