[motowebmaster]

I suppose you can have bots index only your archive pages, and not your forum pages.

There are plugins that will wrap an announcement/notice in a Javascript tag that search engines won't index. I suppose you could make an "obvious" notice that appears only to guests at that point.

IMHO, content should be king - let users decide for themselves regarding registration. I've run sites that had mounds of useful information frequently accessed by lurkers. It may seem like folks are taking a ride, but traffic frequency isn't a bad thing, guest users can be a useful asset.