This is what the word "moot" means, as defined by the New Oxford Dictionary built into all Macs:
Subject to debate, dispute, or uncertainty, and typically not admitting of a final decision.
That definition is probably the exact opposite of what you thought the word meant. Indeed, that is exactly why this post was written. I'm not sure why all people seem to use the word "moot" to proclaim that something is closed for discussion, but somewhere through time the way people used the word seems to have reversed itself. (I'm not even quite sure when, or how, I discovered its true meaning — I was guilty of using "moot" incorrectly for years, myself.)
Even some of the best, most meticulous writers have made this mistake before. Take John Gruber of Daring Fireball, for instance. He's well known for being almost disgustingly meticulous when it comes to nitpicky details of grammar, and yet, when linking to this very site, he misused the word "moot". Pretty funny, no?
Of course, there are those of you that might counter, quoting the second definition of "moot" that reads as follows:
Having no practical significance, typically because the subject is too uncertain to allow a decision.
But, while that isn't the total opposite of how people actually use the word (like the first definition is), it is still not how people actually use the word. People use "moot" when they are very sure about something, and thus have deemed any further argument unnecessary — not the other way around.
Anyway. Just thought you should know. Now go on and be that annoying jerk who's always correcting people's grammar.