For the most part everyone seems to have respected the "no js on checkout" line of thought but recently there's been a string of apps that have popped onto the scene exploiting this loophole and charging a pretty penny for it. I suspect that none of these apps are making any potential risk known, but will of course happily take your cash. I am not expecting those apps to be in the store for long.
If you are a Shopify Partner or Expert don't even think of going down this path. You should know better than to do this.
I don't care about breaking my checkout. How do I do it?
If this seems pretty interesting to you, and you want to give it a try - go for it. That does require that you head back to that first paragraph and gauge if that's a risk you're willing to take.
[update:27/05/15]. If you're wondering if this still works on the new checkout - it does.