If iChat throws up a Feedbag error 10 (what oh what is a feedbag?) its because you’ve got a “Recent Buddies” group in AIM. Remove that with something sensible like Adium, and suddenly, like magic, you’ll be able to add AIM and .Mac accounts again.
It doesn’t seem well documented, but this macnn post seems to be somewhat useful. Is there still a Radar Friday going on?