If you get errors like this in your site header:
Warning: session_start() [function.session-start]: open(xxx) failed: Permission denied (13) in xxx/wp-content/plugins/woocommerce/woocommerce.php on line 80
your server is to blame.
PHP Sessions may not be set up correctly, or the sessions directory (usually /tmp) is not writable. You need to contact your hosting provider to resolve this.