The problem here is that "moot" has two distinctly different meanings, depending on your audience: Americans and the rest of the world appear to treat "moot" differently. A "moot point" (the typical use of moot) was originally one that was up for debate. As Michael Quinion writes in World Wide Words (emphasis added):

It comes from the same source as meet and originally had the same meaning. In England in medieval times it referred specifically to an assembly of people, in particular one that had some sort of judicial function, and was often spelled mot or mote. So you find references to the witenagemot (the assembly of the witan, the national council of Anglo-Saxon times), hundred-mote (where a hundred was an Anglo-Saxon administrative area, part of a county or shire), and many others. So something that was mooted was put up for discussion and decision at a meeting — by definition something not yet decided.

Lord of the Rings readers may recall the "Entmoot," a meeting of the Ents. Tolkien was keenly interested in linguistics and philology, and his use of "moot" reflects Quinion's linguistic understanding above.

What the OED Says

Furthermore, Maeve Maddox reports that the OED's primary definition for "moot" is:

1. Originally in Law, of a case, issue, etc.: proposed for discussion at a moot (MOOT n.1 4). Later also gen.: open to argument, debatable; uncertain, doubtful; unable to be firmly resolved. Freq. in moot case, [moot] point.

But to make things worse, Maddox points out the OED's second definition of "moot," acknowledging its common use in American English:

2. N. Amer. (orig. Law). Of a case, issue, etc.: having no practical significance or relevance; abstract, academic. Now the usual sense in North America.

Similar definitions appears on the Oxford Dictionaries and Merriam-Webster sites.

Moot Court

Moot court is a common activity in law school, in which students prepare arguments and present them before "judges" who are typically their professors or other established lawyers. In moot court, students are exposed to both sides of an argument, and generally argue whatever position is assigned to them. By definition, the issues explored in moot court are "open to debate" (in the sense that students are debating them), but moot court debate is of little overall significance because the moot court case is only hypothetical.

Many writers have suggested that this legal usage of the "moot point" may have led to the current American view of the word "moot" by a chain of logic something like this: a "moot point" is often an issue of little practical significance, assigned for argument in moot court; because the point itself may be academic or irrelevant, it's probably not worth arguing about outside of moot court; therefore a moot point is something of little significance. This is a neat trick of language, and seems plausible to me -- we go from the term "moot" clearly meaning "open to debate" and end up with "an issue not worth debating" (which, for the record, doesn't mean it's a settled point -- it just means that debate won't get us anywhere).

Moot vs. Mute

Another problem with "moot" is pronunciation. Because it's so often used as part of the phrase "moot point" and rarely heard in the English language elsewhere, speakers may assume that the word in question is actually "mute" (meaning silent, or unable to speak). These terms are different, and the pronunciation of "moot" is similar to the word "hoot."

The Take-Away

If you're writing for an international audience, you probably want to avoid the word "moot," because it might mean the exact opposite of what you intend -- depending on who's reading. As an American, I find myself naturally using "moot" in its Americanized Rick Springfield sense ("not worth debating"), but now that I've been exposed to dictionaries, I find it hard to use the term at all, for fear of being misunderstood by a broad audience. It may be simpler to use the term "debatable" when you mean that, or a phrase like "not worth debating" when it's appropriate. The only bummer about those is it's nearly impossible to find a good rhyme for "debatable" in a pop song. "Dateable," I suppose? Oh wait, that's not really a word. Sorry.

Trivia tip: apparently Rick Springfield's first band was called Zoot. I detect a pattern.