The leader asides in Thematic Power Blog v1.0 will invalidate your XHTML if you're using both them and the subsidiary asides. Nothing will explode because of this but it is highly annoying. Here's how to fix it.
Find lines 63, 69 and 75 in functions.php. On each line change "first", "second" and "third" to "first-leader", "second-leader" and "third-leader".
In style.css on line 34 change
#leader #third to
#leader #third-leader. Problem solved.
This will have been addressed in Power Blog 1.1 and you won't have to worry about it.