Several of us in Operations at Yahoo have noticed that over the past few months more and more messages are getting dropped. The thing we have in common is that we all run Mac OSX and using the latest Adium (however we've noticed this in the last view versions). Someone using a windows machine and native Yahoo messenger don't see the messages dropped, but in fact they can send any of us a message and maybe 5% of those messages never arrive. This is obviously a big problem. We have confirmed that it seems to be common among all our mac machines all running Adium x.84 - x.86. Also we notice the problem when sending messages from adium client running on mac to another adium client running on mac (about a 5% message drop rate). So it's my feeling that there is a problem confirming delivery to the adium client and the message simply is dropped.
Please let me know how I might provide more information to help resolve this issue.