R. U. Serious wrote: 1. How are you going to "hide" the links? -> You really can't, only make them less obvious.
2a. Do you want to duplicate each link? -> confusion & pagesize
2b. Only duplicate one link on the index? -> every spidered page will be on level deeper and possibly have a worse rank. Still confusion if normal users entered that part...
3. People coming from searchengines will automatically land on the sid-less forum. What do you do with them?
4. How do you want to pass the information between pageviews at "which forum" somebody is looking? -> Add yet another get-variable to the URL.
Although I am not saying it can't be done, that is not my choice/way to do it.
If I get spidered, I'll just add the user-agent to the my sessions.php and be done with it. A lot easier and hardly any confusion for users.
TO get around all of those obstacles what I envision is simply a "spider page". To force ANY search engine to index all the threads.
The page would be linked to in the overall header invisibly, this page would run a script similar to a "latest posts" mod. It would simply list ALL the threads with links that do not have session ID's .
When threads are linked to in this way search engines don't seem to have a problem indexing them. This workaround would expand on your mod so that people who do thourough search engine manipulating will be able to target other search engines.
Do you think this idea would work?