For what it's worth, the 5.1.5 repository Forums package does seem to work flawlessly in 5.2.0 (though certainly not all 5.1.5 packages do!), including search. I've spent a half an hour or so poking at it and I couldn't make it fall down or give me any trouble. If it were the only package I was using, I would have upgraded to 5.2.0 already.
I think search was easier to install in 5.2.0 than 5.1.5 (but I don't understand what's happening at any point in the process, so I'm just guessing based on what I saw when I was upgrading!), though I'm sure the PostgreSQL functions still have to be installed manually per the documentation. Probably.
