I can see us creating .catch(fn) as an alias for .then(null, fn). That is different than .done(fn) though because the latter adds a new handler to the existing Deferred.

of course, this would need to be done with ['catch'] so as not to trigger reserved word treatment in older browsers.

Giant +1 on this. Since these Promises aren't based on a prototype, it's much easier for this to be supported internally rather than having to do something like wrap the constructor and decorate the returned promise.

I updated the title for accuracy. Suggested implementation:

"catch": function( fn ) {
    return promise.then( null, fn );
}

Sounds like a blocker to me