I'm having the same issue is this due to the PHP 5.3.0 version failing if the session fails to start?
I did the fix but am still getting this error:
Warning: session_start() [function.session-start]: Cannot send session cookie - headers already sent by (output started at /Applications/MAMP/htdocs/wholesale/store/users_Registration_full.php:5) in /Applications/MAMP/htdocs/wholesale/store/WA_eCart/WA_eCart_Definition_PHP.php on line 480
I'm testing locally on MAMP with PHP 5.3.2.