I am interested to see what you guys are doing with regards to this. One way I look at it is to encourage new member registrations by not allowing guests to search forums, however in a way this is alienating their user experience. Some visitors may just want to search for stuff before registering. What's the best solution?
I think that unless your forums are very large and you are not in dire need of more members, that you should let guests search freely. Guests can search just about any forum they want out there...so unless you dominate the industry that you are in or offer some super-unique or valuable information, you should let guests search...otherwise, when they see that they can't search, they're just going to click the little x and go to the next best forum. If there is no next best forum, then it won't hurt you too much to do this.
I do it that way: - Disabled forum search for guests - Included a Google Coop Search attached to my AdSense Account right underneath the Forum Search Box That way guests don't penetrate the server load and eventually make me some $ by searching the forum