If you force people to register to read your forums, you’re going to lose a lot of potential members.
You’re going to scupper your search engine results. You’re going to deter potential members from joining. You’re going to hide the biggest selling point of your community (the conversations) behind a registration wall that the majority wont cross.
Registering to read isn’t the same as registering to participate. You want members to register to participate. If they’re registering and reading, they’re useless. It’s the participation that you need. Let members see the conversations first, before they join.
Bigger point, what is the purpose of registrations at all? Is it a counting procedure? Couldn’t you replace your registration page entirely with Facebook Connect, OpenID or a similar platform?