$.wait.js

// the queue of things that are waited for
var waitQueue = (new $.Deferred).resolve().promise();

$.wait = function(waitfor){
	var dfr = new $.Deferred();

	// poll until a function returns true, then resolves the dfr
	// TODO: throw on timeout
	var poll = function(fn){
		if(fn()) dfr.resolve();
		else setTimeout(function(){
				poll(fn);
			},100);
	};

	// get a function that will 
	// resolve this deferred based on `waitfor`
	var wString = Object.prototype.toString.call(waitfor);

	var fn = {
		'[object Function]': function(){
			poll(waitfor)
		},
		'[object String]': function(){
			poll(function(){ return $(waitfor).size()})
		},
		'[object Number]': function(){
			setTimeout(dfr.resolve, waitfor)
		}
	}[wString];

	waitQueue.then(fn)
	waitQueue = dfr.promise();

	return dfr.promise();
};

.wait.js

// the queue of things that are waited for
var waitQueue = (new $.Deferred).resolve().promise();

$.wait = function(waitfor, callback){
	var dfr = new $.Deferred();

	if(callback)
		dfr.then(callback);

	// poll until a function returns true, then resolves the dfr
	// TODO: throw on timeout
	var poll = function(fn){
		if(fn()) dfr.resolve();
		else setTimeout(function(){
				poll(fn);
			},100);
	};

	// get a function that will 
	// resolve this deferred based on `waitfor`
	var wString = Object.prototype.toString.call(waitfor);

	var fn = {
		'[object Function]': function(){
			poll(waitfor)
		},
		'[object String]': function(){
			poll(function(){ return $(waitfor).size()})
		},
		'[object Number]': function(){
			setTimeout(dfr.resolve, waitfor)
		}
	}[wString];

	waitQueue.then(fn)
	waitQueue = dfr.promise();

	return dfr.promise();
};