A Deferred object should (ideally) provide a mechanism for detection that it's a deferred promise. For instance, take code that calls a callback which may or may not return a deferred promise:

var result = someCallback();
if(result.isDeferred) {
    result.done(function() { // ... } });
} else if(result) {
    // ...
}

Currently, the following fails (for obvious reasons):

var f = new jQuery.Deferred();
if(f instanceof $.Deferred) {
    // ...
}

I would suggest a simple boolean on the object itself. I realize this is no better than duck type detection of the relevant method (typeof result.done === 'function'), but it is syntactically cleaner and easier to read.

Just a thought -- thanks!