Google has a program for webmasters called First Click Free.

To implement First Click Free, you need to allow all users who find a document on your site via Google search to see the full text of that document, even if they have not registered or subscribed to see that content. The user's first click to your content area is free. However, once that user clicks a link on the original page, you can require them to sign in or register to read further. The user must be able to see the full content of a multi-page article. You can allow this by displaying all content on a single page to both Googlebot and users. Alternatively, you can use cookies to make sure that a user can visit each page of a multi-page article before being asked for registration or payment.

Does Bing support anything like this?