Cross-browser event handling

I need a cross-browser function for registering event handlers and a (mostly) consistent handler experience. I don't need the full 开发者_开发百科weight or functionality of a library such as jQuery, so I've written my own. I believe I've accomplished my goals with the code below, and so far my testing has been successful, but I've been staring at it for too long. Are there any flaws in my logic or gotchas that I'm missing?

EDIT 1: Commented each block with browser intent for clarity. Updated IE block to not call func right away (thanks to Andy E's keen eyes).

EDIT 2: Updated IE block to call func.call() with this instead of elem.

EDIT 3: Updated to pass JSLint with "the Good Parts."

function hookEvent(elem, evt, func)
{
    if (typeof elem === "string")
    {
        elem = document.getElementById(elem);
    }
    if (!elem)
    {
        return null;
    }
    var old, r;
    if (elem.addEventListener)  //w3c
    {
        elem.addEventListener(evt, func, false);
        r = true;
    }
    else if (elem.attachEvent)  //ie
    {
        elem[evt + func] = function ()
        {
            func.call(this, window.event);
        };
        r = elem.attachEvent("on" + evt, elem[evt + func]);
    }
    else                        //old
    {
        old = elem["on" + evt] ? elem["on" + evt] : function (e) { };
        elem["on" + evt] = function (e)
        {
            if (!e)
            {
                e = window.event;
            }
            old.call(this, e);
            func.call(this, e);
        };
        r = true;
    }
    return r;
}

There's a problem on this line:

r = elem.attachEvent("on" + evt, func.call(elem, window.event));

This will execute func() immediately, instead of attaching it as a handler for the event. Instead, the return value of func() will be assigned to the event, which will throw an error if it's type isn't "function".

I can understand that you don't want to use a framework, but many (many) others have written cross-browser event handling snippets. John Resig has one version, Google for "javascript addEvent" for many more.

http://www.google.com/search?q=javascript+addevent