I was looking at the Daily Halachah Yahoo group and was a little bit surprised at this statement:
It is therefore forbidden for Jews to teach non-Jews Halachah
Is this true?
(See a related question on the Meta).
Even if it is true, it wouldn't be a concern on this site, because the [primary] purpose is for other Jews to see it. Even if we're answering to a non-Jewish user (which of course, is impossible to tell) there are still other Jews who will see it and benefit from it.
To give it a real world analogy:
Publishing a sefer would have the same problem. Any non-jew can pick up a sefer and read it...
And for an actual source: Rabbi Mechanic tells the story of how he once had to speak at a class about religion, where only one of the class attendees was Jewish. His stipulation for coming to speak was that the one Jew in the class show up that day. One of the reasons he gave for that stipulation, was to get around the problem of teaching non-Jews Torah.