Using yui3@0.5.34 and yui3-core@3.4.0 on NodeJS 0.4.5, output is:

{"rss":26628096,"vsize":3125485568,"heapTotal":18382208,"heapUsed":8969544} - initial
{"rss":48410624,"vsize":3133837312,"heapTotal":51860896,"heapUsed":27612624} - after 10000x Y.before()
{"rss":36372480,"vsize":3141439488,"heapTotal":26666400,"heapUsed":24034864} - 10000ms later

Notice how the heapUsed remains high even after the 10 second timeout to allow the GC to run. The sample program:

var YUI = require("yui3").YUI,
iterations = 10000,
timeout = 10000; //millis

function showMem(msg) {
console.warn(JSON.stringify(process.memoryUsage()) + "t - " + msg);
}

YUI().use("event", function(Y) {
showMem("initial");
for (var i = 0; i < iterations; i++) {
var object = {
original: function() { console.warn("original"); }
};

Y.before(function() {
console.warn("before");
}, object, "original");
}

showMem("after " + iterations + "x Y.before()");

//wait for GC to run
setTimeout(function() {
showMem(timeout + "ms later");
}, timeout);
});