C# - Dynamic Master Pages In SharePoint 2007 Publishing Sites?

I'm trying to do dynamic switching of the master page in SharePoint 2007 publishing site.

I'm following this example which uses a HTTP Module [URL]

Here is my code

public class SwitchMasterPage : IHttpModule
{
public void Dispose()
{
}........

Everything is working fine when the current page type is an Application Page, however when the page type is a Publishing page (e.g. BlueBand.master) the page_PreInit procedure is never called - it is still being registered with the event handler though.