Arg, not this again.
There are far more examples of armed citizenries endangering or destroying democracies than there are of armed citizenries successfully protecting democracies. Just read through our little discussion on Flanders and Wallonia, and imagine that Belgium was suddenly flooded with arms and "well-regulated militias," Walloon and Fleming. The country would go up in flames in no time.
For a real-world example, just look at Lebanon -- the closest the Arab world has come to an open or democratic society. They hate each others' guts, but what makes it dangerous is that everybody is armed too.
And finally, given that the government is equipped with nuclear weapons, heavy armor, SEALs, and an air force, individual armed citizens, or even citizens organized into militias, don't really count for much. (See our discussion on the American Revolutionary War for more on the "200 years ago" discussion.
For the record, I am not against gun ownership. There are plenty of perfectly stable countries with very little gun crime and lots of guns. If Americans can't handle their firearms, the reasons lie elsewhere. It's just that this particular pro-gun argument is pretty damn fallacious.