If you look at Apple's forums, they have a box that welcomes visitors to ask a question. If they are not logged in, they get directed to a login/register box. Anyone who takes the time to type a question is probably more likely to take the time to register. It also makes the process of that first post incredibly easy. Once asked, perhaps forum admins can push a list of forum categories, so that a next step for the user would be to select the appropriate forum category. That or perhaps it goes into moderation.