Geoff,
Some people have been saying some misinformation to you, maybe because they're not fully reading or comprehending what you are saying.
YES! iMessages will use FaceTime if you click the video button in the messages area.
iMessages only can: FaceTime, Text, file share.
iMessages does not use the features that are normally attributed with iChat or Instant Messaging in general.
For those features, you will need to log into Gtalk, AIM, Jabber, Yahoo!, or Bonjour.
once you sign into those, you will have access to a buddy list, status messages, Screen sharing, 3 way Video Chat (with the fun little animations and effects from photobooth), and 3 way audio chat.
If you connect with Facebook, sadly, it does not support these features, even if your other friend is using Messages as well.
In a nut shell, Yes, iMessages uses Facetime to video chat, and is not associated with a buddy list or Video Chat through Messages.
Messages ADDS the iMessages client to OSX (this is a huge get for anyone with an iOS device). HOWEVER! Messages can still be used like an instant messaging client as long as you sign into an instant messaging client that will support the features you want.
In no way does the added feature to have iMessages in Messages hinder the ability to sign into AIM with your Apple ID and use it like you used to in iChat. The only way you could use your apple id in ichat was through aim. Now you have the ADDED benefit of using iMessages to sync with your's and your friend's iOS device.