Just got a fatal error related to Pro Sites

Team WPMU,

I just updated some plugins this morning, one being Pro-Sites and another being AddThis Social Bookmarking Widget. When I tried to activate AddThis, I got this error:

Fatal error: Cannot redeclare is_pro_user() (previously declared in /home/thirdsc3/public_html/wp-content/plugins/pro-sites/pro-sites.php:3843) in /home/thirdsc3/public_html/wp-content/plugins/addthis/addthis_social_widget.php on line 2450

These plugins have played very well together for over a year now, so I was quite surprised to see this error. Any ideas? Add This is a very important plugin to my users.