Thanks to Scott Hanselman for pointing to this beauty:
Josh Twist over at JoyOfCode has a “Validator Module” that sits at the very end of the HttpRequest and validates the page you just rendered as XHTML or not, then displays the detailed results. If you’ve ever found it a hassle to run XHTML validation on your site at runtime you should take a look at this clever idea. With tools like this ValidatorModule plus validation within Visual Studio.NET always improving as well as tools inside of FireFox to validate, there’s fewer and fewer excuses to not render XHTML.
Try the asp.net2.0 XHTML validator out on Josh’s site, pound on his demo, and leave him some feedback. You can add it to your .NET 2.0 applications without recompiling. He’s also expressed that he could backport it to .NET 1.1 if there’s demand.