You live and learn. I always thought "Let's don't" was a joke, something you say facetiously to indicate that you do care about grammar. It turns out that "Let's don't" has arrived in that grey area between grammatically right and wrong where many consider it just non-standard, but not wrong:

http://en.wiktionary.org/wiki/let%27s

What I find really interesting is that in British English, it's "Don't let's." How does that happen? But we digress.