Messenger Plus! wasn't planned on being run on a Mac. Patchou, at the time was the sole developer and didn't have time to try and learn the architecture to a Mac.
Patchou's reply to Mac Users ?
Now with Patchou out of the picture and a group of developers working on it you never know what could come of it.
Keep in mind the API (which is how Plus! integrates with Windows Live Messenger) doesn't exist in a Mac environment. So even if this idea was on the radar you wouldn't see it any time soon.
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