i get what u mean
basically phpbb doesn't care where it is. all data is retrieved from the database and subdirectories are accessed relative to the forum's main directory.
so you can install the forum in /forum/ and then rename it or move it to whereever you want and it wouldn't matter.
so if you went in cpanel and made a subdomain called BLAHBLAH, then i think a subdir called BLAHBLAH will be created in your main public_html dir. you can then move the stuff inside /forum/ into the BLAHBLAH (making sure it's doesn't become BLAHBLAH/forum/ but just /BLAHBLAH/)
after that if you go to BLAHBLAH.****** , it is actually accessing www.******/BLAHBLAH/
i think that's how it works.. or that's how i did it before..