We have one of those, too. The people sitting around me and I call him "Hypo-Man: Avenger of Would-Be Scenarios." We drew him a little Superman-style shield and everything with a very impressive "H" instead of Superman's "S."
Hypo-Man... To the rescue!
He actually got called out a few weeks ago by our Crim Law professor, but was too socially inept to realize it, and continued on with the same line of idiotic questioning in our other classes.
I blame the professors for not being direct enough with these people. If they would just say, "Hypo-Man, that's irrelevant. Let's move on," we'd all get a lot more done.
There are few "What if" people in my class, but for the most part, Hypo-Man is the most consistently annoying, followed by the girl who has to paraphrase back everything the professor says to make sure she is "understanding it right."