I saw the idea of merging objects to overwrite default options some time ago in a plugin...

Many plugins have default values that can be overwritten by the client. A convienent approach is to use an object literal to define defaults and merge the defaults with options given by the client.

Example:

defaults = {
  option1: 1,
  option2: "foo"
}
options = {
  option2: "bar"
}
// merge these
$.mergeObject(defaults, options);

// Would result in options being unchanged an
// defaults with option1 equals 1 
// and option2 equals "bar"

The mergeObject function is very small:

/**
 * Merges two objects, overwriting all properties
 * of the defaults with properties from
 * the options, if present.
 */
jQuery.mergeOptions = function(defaults, options) {
	for(var i in options) {
		defaults[i] = options[i];
	}
}

Having this in jQuery core and adding some documention about this could by quite a help for plugin developers.