Not too bad. I ran my new theme from last night through the W3C Validator quickly this afternoon and there were only 28 errors initially. I forgot a few required attribute elements to some of my tags and had an extra unused unordered list tag set. Other than that I merely had to use some htmlentities tags around my titles and alt attributes and my 28 errors are now down to zero.
Woot for being XHTML 1.0 Transitional with a WordPress site!