Oh no. AOL Instant Messenger is totally free. The actual AIM client (there's one for X) has ads on it. You can communicate with people who actually use AOL as their ISP as well as the millions of people who only use AIM for chat. Basically, iChat is an alternative chat client that taps into the same AIM network. So even if you don't use .Mac, you can sign up for a free AIM screen name and use it that way.
I too have a couple of iChat questions for those of you with developer's previews...
1) How do the user pictures work? Is it basically like Buddy Icons where you pick your picture and it automatically sends it to your buddies? Do you have to assign pictures to your buddies, or do you automatically receive whatever they've chosen?
2) I'm pretty addicted to Adium and tabbed chatting... how does iChat measure up?