We're going to put this under rumor for now, though certainly plausible.
Kevin Marshall for Clarity Consulting, a group very familiar with Windows Phone (hint: the Facebook app), has posted an image via his Twitter account showing what looks like a Google+ app for Windows Phone. The hashtag is #wp7, the phone is an HTC Trophy and it clearly says Google+.
Of course with no other info, it's hard to tell if it's real or not--after all, it's not hard to put an image on the phone and call it an app. Still, Marshall doesn't seem the type to fib on the internet either. So...interesting.