meaning - What does “double-down” mean when used in a context other than Blackjack?

If we consider the context

Several readers point out another key difference between Bachmann and Palin — their approach to criticism. Bachmann, at least so far in the campaign, has shown a willingness to acknowledge mistakes she makes on the campaign trail. Palin is notorious for doubling-down on perceived missteps, typically blaming a biased media for not telling the whole story.

I think the meaning is clearer. Palin not only makes political mistakes, but then further compounds those blunders by blaming the media for misrepresenting her, instead of admitting that she said something nonsensical about Paul Revere.

It's a gamble to blame the media, not only because the media is the outlet for her message, but also because if people don't believe the media is to blame, then she looks even worse.

To answer your other question, I have occasionally heard double-down to refer to a situation where someone takes a bold risk when they are already gambling. For instance, I had not studied for an exam and I doubled-down by sleeping until fifteen minutes before class.