Routing error using MVC on iis 5.1

Question

I am using .NET MVC 2 with Windows XP (IIS 5.1). I have set up wildcards per this blog.
The problem I run into is sometimes when I call some of my actions, they are returning 404 errors. Here's how I'm calling them (using a button):

            <% using (Html.BeginForm("MyController", "MyAction")){ %>
        <input type="submit" value="Submit" />
        <% } %>

I'm thinking it might have to do with my buttons, but I'm not sure. My actions are accepting Posts only. Any ideas?