Currently Joomla! 1.5 is not supported.
v2 final is approaching... just waiting on language file updates and some small feature enhancements to be finished.
Next goal is to add J1.5 support.
Currently on my test setup I did run a test and after changing 1 phpbb3 file I was able to get CB Connector to run in J1.5... with the exception of the Control panel for sync'ing users... this form still manually loads joomla core files and as such is still trying to load the old J1.0.x files. Luckily my code is fairly modular so I can hopefully change 1 file and have the backend working (note "hopefully" as I have not tested this part yet)...
utf_tools changes are here:
cbconnector.com/forums/viewtopic.php?f=12&t=253#p1323
These changes are needed on systems <php 5.2.4 (IIRC) because both Joomla 1.5 and phpbb3 now use utf_8 and they both recreate php5 functions if in an earlier php... these mods make the phpbb3 files check if the functions exist also before recreating... i.e. if in Joomla then don't create... if in phpbb3 only then create...
Then I plan to ride the native J1.5 wave following behind future CB releases.
Probably more than you wanted to know
Greg