I'm probably jumping the gun just a little here, but I'm pretty excited and wanted to announce that I've been able to achieve Xhtml Strict validation on http://www.snapsis.com using PageBlaster.  Snapsis.com is currently running on DNN 4.3, but this solution will allow you to achieve validation in any version of DotNetNuke and also any other ASP.Net website.

It wasn't easy creating the replacement rules, but now that they are done it is very easy to implement this filter.  Only one change will need to be made to your DotNetNuke portal and that is an easy upload of a modified text/html module. This is only if you have a newer version (3.3) of the Html  module with the edit in place capability.  I'll work with the rest of the core team to get this requirement removed and to also make some other adjustments in the core that should eliminate a few of the replacements that PageBlaster has to do. 

I'll be working today on getting the free version of PageBlaster to run these replacement rules and will soon be releasing an enhanced version of the PageBlaster module for DotNetNuke that will give you even more control of the content delivered through your websites.