Passing arguments to handlers

Not as far as I know. What you can do, though, is have the function you pass as the handler be created in teh context of an object that maybe has state; you can then refer to that state. For example:

someControl = function(element){
someControl.initializeBase(this,[element]);
this._privateValue = 'Hi there';
}

someControl.prototype = {
initialize : function(){someControl.callBaseMethod(this,'initialize'),
dispose : functio(){someControl.callBaseMethod(this,'dispose'),
handleClick : function(ev){
alert(this._privateValue);
}
}

Type.registerClass('someControl',Sys.UI.Control)

// later...

var ctrl = new someControl(element);
$addHandler(btn,'click',Function.createDelegate(ctrl,ctrl.handleClick));

I haven't tried it, but you might even be able to do all that w/o making a formal 'class' since classes are just special functions... you might be able to get away with just attaching the value and the function handler to some random js object.