In phpBB 3 , you there is already a way to do this without allowing authentication . its called bot access .
If you click that link given in the first post it should load for you and it just shows the page with the site authentication but no actual content yet. SCREENSHOT -
Iv got it.. It's very easy to find. Login to your account. Click on the tab labeled " AdSense Setup " Now when on that page, you should see this menu " Products | Color Palettes | Channels | Competitive Ad Filter | Site Authentication " It's right there on the end!
Well, i've just had a notification on the JoelComm newsletter, and it's two months now since the first posts about it on here, but I've still not got it in my Setup.
I dug around a bit for this and have found that i've had to upgrade by login to a Google account for this to appear. I've set it up for my forum, but since then Google hasn't been back to my site :-(