I've just upgraded from 2.8 to 3.1 and am also having this problem:
Fatal error: Call to undefined function: get_regcode() in /xxxx/xxxx/public_html/reviews/pp-inc.php on line 1695
Here's my Reviews page:
http://www.photographyblog.com/reviews/
I've tried deleting ib2.php and re-uploading it several times (that's the forum software that I use).