And you would probably be right.
But what about a cloud like the one pictured below, perhaps seen from a distance?
Lenticular clouds like this one (and others you can see at this really nifty site - a few remind me of Cylon baseships!) can indeed be mistaken for a UFO - particularly of the very large, slow moving "mothership" kind.
Does that mean that all UFOs are clouds?
Of course not. It just means that the idea that some UFO's are clouds is not as silly as it might at first seem.
Addendum: By the way, New Scientist had a short article with the same title, about the same subject, back in October. I didn't know that when I first posted this - a weird synchronicity.