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explains a little of what I've done. Since it's a glaring security hole, I'm reluctant to post my *actual* code: it only checks on IP address, not on user agent right now, and I think it might be spoofable; and it would reveal the session ID I've used for Googlebot. However, it ought to give you a few ideas about how you'd achieve this for yourself.
According to an e-mail from Google tech support, while this is technically cloaking, this is acceptable behaviour, and they gave me IP addresses and user agents to help me in my quest (which you'll find in this forum).
I can report that it has worked well in the past; the fake session has been deleted though. I suspect this is because it has logged itself out somehow - it does follow all the links on the page. I'm now writing a bit of code to allow the system to recreate this session.
What james has posted is a helpfull hint how to let mediapartners bot crawl your private phpbb forums and let it detect the topic of the private forum pages - to be able to publish relevant AdSense ads instead of PSA's.