I run various antispyware software regularly. One of them is Xoftspy. Their latest definitions update includes Messenger Plus! as unwanted spyware:
http://www.paretologic.com/resources/definitions....=MSG%20plus%20Live
Of course, I'm not afraid of Plus!, I think it's a great program and gave me the option to refuse sponsor programs during install, so I'm surprised by the above rating it received.