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I finally just got mine working just now.. for yahoo mobile IM.. at the login screen on the bottom left is an options button.. then preferences .. i clicked to "reset applicaton".. which cleared the history. .user name etc off the phone.. i just had to re-enter my login info.. and it worked right away. not sure if this works the same for AIM or MSN etc. but did the trick for yahoo. Please note you will lose the saved conversations if you have any that were stored on the phone etc.. and any settings you might have changed. Hope this helps 
Happy New Year!