A lot depends on how “big” your phpBB forum is expected to be.
I currently manage a friend’s site that is dominated by the phpBB forum, and I will soon be converting his static HTML pages, including the Home Page, to WordPress. I already have a test version of his home page that includes the same View Most Recent Posts display.
Note to future readers: the links that follow were valid on September 9, 2009, and may not be as soon as October:
Static HTML home page: http://www.radiowest.ca
Wordpress test home page: http://www.radiowest.ca/wp/
(needs some cleaning up)
Note that both of these pages have the View Most Recent Posts area generated by (two different) WordPress installs. The static HTML one uses an iframe to include it.
To state the obvious: the first thing you will need is a link to the Forum. Not sure how your menus are built. If you cannot easily add the Forum to the menu, you can just add a Link someplace else on the page.
It is relatively straight forward to add your logo to the phpBB header. Ads are more problematic. My friend and I chose to add banner adds to the bottom of every forum page. As well as chewing up less screen real estate at the top of the page, it also seemed less objectionable to our visitors.
Take a look at the top and bottom of our phpBB forum’s home page: http://www.radiowest.ca/forum/
