I haven't tried putting my forums into an iframe, because when I researched forum integration, it seemed like a lot of people had trouble with the iframe method.
Instead of creating an HTML page and trying to put phpBB inside of it, I did like many others have done and "wrapped my site's look and feel around my forums," by adding the HTML code of my site's existing header, left navigation column, and footer, to phpBB's overall_header.tpl and overall_footer.tpl files (which you can find in
templates/tamplatename/).
AFAIK, the only downside to this approach is that you have to edit the overall_header.tpl and overall_footer.tpl of each style that your forums use (mine only use my own customized style).
Here's how it looks:
http://www.computerbob.com/forums/