They've recently published a very cool XHTML Validation module for ASP.NET 2.0 that allows you to easily identify and catch any non-XHTML markup with your application. You register it as an ASP.NET HttpModule within your app's web.config file, and then it watches and checks all page output for XHTML compliance:

ref:
http://weblogs.asp.net/scottgu/archive/2006/04/09/442336.aspx
download:
http://www.thejoyofcode.com/default.aspx