Amazed that Google permit that, but oh well. I guess they take things on a case-by-case basis.
Myself, I would prefer NOT to have my private forums have any "loophole" that could be abused...
Anyhow, in response to your question. Yes, I have noticed the same as you, with header & footer (especially metatags). However I have chosen not to remove them, in part for the following reasons:
- Rather than displaying no ads (showing PSAs or alternate ads) on pages which do not have much content, I get a "default" ad set that's based off my keywords. I also get this default ad set once mediabot has visited the page for the first time - usually it makes more relevant ads available soon afterward.
- I can't see it sitting well with Google.. if they wanted to ignore keywords etc, they will modify mediabot accordingly. Even if your intentions were honourable, it really is something they're well aware of, a nd choosing to use as a "guide" to the intended content of a page. So make your keywords relevant.
- I believe that both keyword changes on my site, a revisit by mediabot, and perhaps a mediabot upgrade (don't know for sure) resulted in what is now only the occasional phpbb/forum based ad (usually attributable to the keyword "forums" rather than phpbb at the bottom of the page) :-)
By all means contact Google to see about this, but I think their answer is likely to be no. They're pretty reasonable, but I think they'd prefer to just be served the same page as users as much as possible..