I am really against the idea of asking counterfactual questions in the class room. The answer to: What about have happened if Hitler had won World War II, is completely irrelevant and should not be addressed in the class room. Students have a hard time as it is understanding how and why history happened without introducing historical alternatives into the mix. What I think is most offensive about counterfactual history, however, is that it assumes that history is not exciting, interesting, or there are no good meaty discussions that can be held about a subject unless you introduce What If questions. Historiographical questions, questions of significance, or relative importance can all be as interesting and definitely more enlightening than counterfactual questions.
I mention this because a software company is coming out with a counterfactual historical game for the classroom. While historical software games on their own can be fun and a good alternative to Doom, I think it is a real mistake to bring such tactics into the classroom. History teachers should not pass the torch to computer games on making their subjects interesting. There are other ways to catch the attention and interest of students.
A good source for more information on counterfactual history -- although it does favor the use of counterfactualism can be found at: Counterfactual History is Bunk.