MySQL does not support unicode before version 4.1, to get unicode support with eZ publish you need to run PostgreSQL or run an alpha version of MySQL. You would need to configure MySQL 4.1 manually as we've not added support for this in the setup wizard.
BTW: which version of MySQL are you using with phpBB2?
It might be that utf-8 will work on MySQL even without native Unicode support, it really depends on what kind of text handling functions are used eZ publish. For instance matching one whole utf8 string against another does not require utf-8 specific functionality, it's actually one of the features of utf8. The places where I see trouble are with searching.
It could be an intereseting experiment and see if eZ publish would run in utf8 mode on MySQL.