Is this a good idea? I realise it's more likely to get users to signup, but will it have a negative impact on SEO? For example, if you're required to be a registered user to view the threads, does this also mean bots for Google, Yahoo and the likes cannot index that content? I'm tempted to switch to this but I'm unsure what SEO effect it will have, if any.
Yeah, it would limit SEO. I know there is a mod out there on vbulletin.org though that limits guest viewing to a specified number of pages. It also gives full access to Google bot i believe. It might be worth looking into.
Bots can't do magics, unless you submit thread content as a sitemap... Bot is identical to a system user. How bot see is what you see as a human?
Bot's aren't identical because the user-agent is different. Some modifications can detect the browser's user-agent, and grant permissions if it detects it's a bot where a normal user is denied.