Session start error

I'm in the process of developing a sell page for my multi-site installation. I'm using your Business Feature theme and have ProSites enabled. All are at the current latest versions.

When I set an option for Appearance->Theme Options->Front Page->Select a Category for your front page, I am getting a PHP error:

Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at /site_path/wp-content/themes/business-feature/header.php:2) in /site_path/wp-content/plugins/pro-sites/pro-sites.php on line 682

Thanks for your help.