I'm doing the same thing, and the problem that I've run into is that pretty much every progress bar on the Internet is written in PHP. This wouldn't be an issue, but the MVC project I'm working on is hosted on a server that doesn't support PHP. Basically, we have to replace the 'script' path with one that will resolve to a Controller action. Since a controller is a compiled class, and the argument is expecting a script url, this is a problem. I was working with the jQuery File Tree plugin, and it uses an aspx page with a script on it as one of its alternate connectors. Using this template as an idea, I tried the same with Uploadify. I put together a page that looks like this:

<%@ Page Language="C#" AutoEventWireup="true" %>
<%@ Import Namespace="imported.namespace" %>

<%<br /> someController ac = new someController();
ac.Upload();
%>

All of the logic for processing the uploads has already been written in my controller class, and I just need a progress bar, so I thought this might work. Unfortunately, I received an Http error, and as yet have not figured out what caused it. It could possibly be due to the fact that I'm processing form data in the Controller code. The reason I think that is because I tried using an Html.ActionLink helper to create a button, and my form data was null when I used it. I had to use a 'submit' button to process the form data. If something similar is occurring here, I don't know how to fix it yet.