Problem solved - I think. Deleted ALL com.apple.iChat files in preferences, not just a specific one or two of them. Now all buddies are back and I can add new ones.
I tried that yesterday, and it had no effect. Deleting all of them today seems to do have done the trick. I use iChat very infrequently so I won't know if other iChat features are working for a while.
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I've lost my Buddies!
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