It looks like Tapatalk has to be enabled specifically by a forum administrator, and they don't even make the plugin to do that public - you have to write them for permission. And I'm not inclined to go to that much trouble for a paid app - we'd be better off spending the same amount of time rolling out a dedicated iPhone-specific skin.
There seem to be a number of apps designed to read phpBB forums like ours, though, so if you search around a bit you might find another one (perhaps even a free one) that automatically works with standard forum software packages like phpBB.