Are you using Peerguardian2? I heard that it may cause these type of errors, and a complete (erase and install) os reinstall may be required to correct.
I had this experience after upgrading to snow leopard, and now, having done the reinstall and no longer having pg2 installed, iChat is working well.
I've been having the exact same problem. Called Apple and they said there was a server problem that was being worked on. No estimate for when it would be back up again.
I have the same problem, tried to use the ichat today but it didn't work with me all what i'm getting is a massage: "ichat cannot communicate to the mobileme account. General error, please advise me