That wouldn't be useful except for debugging. You can already do this if you also include the compiler. You'd never want to include uncompiled coffeescript in production, it can't be minified to the extent that js can.
What exactly do you mean by "can't be minified to the extent that js can"? Why is that? CoffeeScript files are usually about 1/3 smaller than corresponding JS to begin with.
Well, I meant that whitespace is significant in coffeescript whereas it isn't in javascript. You have a point though, coffeescript isn't nearly as wordy as javascript so it's much less code.
The impact of whitespace on size has to be almost eliminated with gzipped transmission, too - it's about the least entropic content in your code.
I just think it'd be nice to have another scripting language for the web after all this time of JS being the sole king. And I think CS would be great because it's still very similar.