i get that in iChat too! i think it only started since AV but i'm not sure. just looks like something to do with the screen draw. maybe interfering with the dock or something. i've seen similar things happen in Windows.
This happened to me also. It bugs the hell outta me!! I can hardly see the smilies because of that strange plaid pattern. It even came back after a restart, and I don't know how to fix it or what causes it. I agree with tjwett though .. it's probably something that has to do with screen drawing. And I have also seen similar effects in Windows..