Yes, you can make a WordPress, Expression Engine, Textpattern, MovableType, etc. blog COMPLETELY validate. Example:
http://www.christianmontoya.com/
You can even make a WordPress blog (and probably the others) output valid HTML 4 instead of XHTML. Tutorial:
http://www.christianmontoya.com/2006/02/13/serve-your-weblog-as-html-401/