When limiting user access to some pages by using php and mySQL (as you can to in Forums like vBulletin oder in CMSs like PHPNuke), search engines will allways be excluded.
Is there some way to allow special domainnames to access the database?

In my case I'm having a CMS. To get access to the pdfs and tutorials on the server you need to log in. Now I still would like to have google index those PDFs, so that will find those documents at google, but needs to register to download them?!

Any hint?