WP2.7 fresh install does not validate (validator.w3.org). The problem is in core code.
Validation Output: 1 Error
1. Error Line 115, Column 143: end tag for "ul" which is not finished.
This occurs with the 'recentcomments' widget which emits a 'ul' tag (id="recentcomments") however there is no unordered list (because there are no comments), which is an error.
bug is in wp-includes/widgets.php
The ul tags should not be emitted if there are no recent comments.
Likeky there are other similar cases of empty lists with ul tags which should be examined.