Chrispcritters - 9:49 pm on Jul 8, 2010 (gmt 0)
I recently moved a 10 year old authority site out of a CMS package.
Changed the order of the html code (nav moved from top of HTML to bottom), changed every URL (301'd every page), went extensionless, cleaned every single page's html to be strict (no errors at all), most pages saw changes in title and description, and saw no drop in ranking.
You mention there are "minimal" html strict errors. You might want to fix those. Have you noticed any changes in google webmaster tools? Did your css id/classes include your keywords? (over optimization?)