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.
"Don't let's" is grammatical - "do not let us" is the opposite of "let us". A bit old-fashioned, though.
1) Streams branch and merge all the time.
http://www.mightystreamradio.com/PHOTOS/STREAM%20PHOTO%202.j...
If anyone's bastardizing the language, it's computer scientists being overly restrictive with their allusions.
2) http://en.wikipedia.org/wiki/Muphrys_law
> But please, let's don't call it a stream.
"But please, let's not call it a stream."