If any one of you just updated to the newer version of WordPress, make sure that you also update to the latest version of the plugin.
A fellow blogger encountered this error on WordPress 2.1.x yesterday, as he still had the version 2.1 of the plugin.
This is caused by the removal of generate_page_rewrite_rules function in WordPress 2.1
I checked the 2.1 version of the plugin and indeed it is calling to the function at line 43. Version 2.2 of the plugin had this removed.