Joomla does not load UTF8 libraries by default, but some Joomla components do load these libraries. This can cause a fatal error, as phpbb3 always loads the libraries and libraries can only loaded once otherwise you see the error: Fatal error: Cannot redeclare utf8_strrpos
The solution is simple, a check can be added to the phpbb3 UTF8 libraries that checks to see if UTF8 is loaded and only require these libries if they are needed. This extra check will prevent fatal errors when running some Joomla components.
JFusion has already done all the work for you and all you need to do is download the patched UTF8 library file: http://www.jfusion.org/downloads/phpbb
and replace your phpbb3 /includes/utf/utf_tools.php file
Joomla 1.6 update Joomla 1.6 now loads utf8 libraries by default in JString and does not allow JFusion to instruct it to use phpbb3 libraries instead. This results in a fatal error, this can be resolved by applying the following patch: Comming Soon (was lost in transition)