I havent had that issue, I use Windows Live/MSN Messenger on my Aria and signing in isnt an issue. As for being able to use it over wifi, this has been a long standing "issue" in that AT&T doesnt allow it on their devices, the IM client must use the 3G network.
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I use Live messenger also. I suspected this was due to an ATT restriction. I use the phone at home 90% of the time, and being out in the boonies, can only connect on Edge, which is why I want to use our home wifi.
I have found that a way around the restriction and can use Messenger with wifi now.