Anyone that has customised SharePoint master pages has no doubt come across the problem where system pages won't use your nice new master page, but rather use the application.master page instead.
The problem with customising the application.master page is that it will affect all sites on your web server farm, rather than just a specific site.
One way to fix this problem is to write an HttpModule that will intercept the calls to these system pages and replace the application.master page with your sites custom master page.
This article explains how to do it:
http://www.sharepointblogs.com/dwise/archive/2007/01/08/one-master-to-rule-them-all-two-actually.aspx
posted on Tuesday, November 13, 2007 3:38 AM