Limit Viewing/Allow Spidering

[dynamite]

I would like to make a request for the the ability to limit Guest to viewing of the thread titles only. When they attempt to view the actual thread, they will receive a message stating that they must register to view posts.
On the flip side, I still want spiders like Google to be able to fully spider the site so I can continue to draw traffic from the search engine.

The reason for this request is because my bandwidth is greatly increased by users who aren't even participating on the site. Most come to view certain information contained in a thread, get that info, never return, and never participate. I figure if they have to take the time to register, verify their email, etc., then they might actually take the time to participate.

Thanks for any help you can give me!