Changing website favicon dynamically

Here's some code I use to add dynamic favicon support to Opera, Firefox and Chrome. I couldn't get IE or Safari working though. Basically Chrome allows dynamic favicons, but it only updates them when the page's location (or an iframe etc in it) changes as far as I can tell:

var IE = navigator.userAgent.indexOf("MSIE")!=-1
var favicon = {
    change: function(iconURL) {
        if (arguments.length == 2) {
            document.title = optionalDocTitle}
        this.addLink(iconURL, "icon")
        this.addLink(iconURL, "shortcut icon")

        // Google Chrome HACK - whenever an IFrame changes location 
        // (even to about:blank), it updates the favicon for some reason
        // It doesn't work on Safari at all though :-(
        if (!IE) { // Disable the IE "click" sound
            if (!window.__IFrame) {
                __IFrame = document.createElement('iframe')
                var s = __IFrame.style
                s.height = s.width = s.left = s.top = s.border = 0
                s.position = 'absolute'
                s.visibility = 'hidden'
                document.body.appendChild(__IFrame)}
            __IFrame.src = 'about:blank'}},

    addLink: function(iconURL, relValue) {
        var link = document.createElement("link")
        link.type = "image/x-icon"
        link.rel = relValue
        link.href = iconURL
        this.removeLinkIfExists(relValue)
        this.docHead.appendChild(link)},

    removeLinkIfExists: function(relValue) {
        var links = this.docHead.getElementsByTagName("link");
        for (var i=0; i<links.length; i++) {
            var link = links[i]
            if (link.type == "image/x-icon" && link.rel == relValue) {
                this.docHead.removeChild(link)
                return}}}, // Assuming only one match at most.

    docHead: document.getElementsByTagName("head")[0]}

To change favicons, just go favicon.change("ICON URL") using the a

I would use Greg's approach and make a custom handler for favicon.ico Here is a (simplified) handler that works:

using System;
using System.IO;
using System.Web;

namespace FaviconOverrider
{
    public class IcoHandler : IHttpHandler
    {
    public void ProcessRequest(HttpContext context)
    {
        context.Response.ContentType = "image/x-icon";
        byte[] imageData = imageToByteArray(context.Server.MapPath("/ear.ico"));
        context.Response.BinaryWrite(imageData);
    }

    public bool IsReusable
    {
        get { return true; }
    }

    public byte[] imageToByteArray(string imagePath)
    {
        byte[] imageByteArray;
        using (FileStream fs = new FileStream(imagePath, FileMode.Open, FileAccess.Read))
        {
        imageByteArray = new byte[fs.Length];
        fs.Read(imageByteArray, 0, imageByteArray.Length);
        }

        return imageByteArray;
    }
    }
}

Then you can use that handler in the httpHandlers section of the web config in IIS6 or use the 'Handler Mappings' feature in IIS7.