I too get something similar.
Have upgraded 2.05 to 2.5. The Admin side displays fine but the front side shows the entire page then under the footer:
Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at /home/booksfor/public_html/wordpress/wp-content/themes/mistylook-101/header.php:9) in /home/booksfor/public_html/includes/functions/sessions.php on line 97
then it displays inline the the same page again but in the default theme…
Any Clues?