Aha.
Maybe part of the problem for me is that your whole top menu (home | ---> learn | ---> forum | ---> gallery --->) is not visible **at all** in my usual browser, which uses the IE6 engine. This is perhaps to be expected, as you have the menu links commented out in your HTML:
<!---- <li><a href="http://www.pictureaweek.com/words/">home<span></span></a> |</li> --->
<!---- <li><a href="http://www.pictureaweek.com/words/photography-articles/">learn<span></span></a> |</li> --->
<!---- <li><a href="http://shutterweek.com/forums">forum<span></span></a> |</li> --->
<!---- <li><a href="http://www.pictureaweek.com/words/wpg2">gallery<span></span></a></li> --->
It's visible in Firefox 2 though, so it seems you are doing something weird with your HTML comment tagging that FF tolerates but IE doesn't. Safari also ignores the top menu, BTW, so your HTML needs to be fixed up here I think.
Using Firefox I can see that Punbb is indeed in there under /forums, so sorry about saying it isn't in there. It's just likely to be invisible to many people.
Also under Firefox, when within the forum the top menu doesn't sit properly horizontally - it is displaying on two lines for me: there is a line break or something just in front of the 'forum' menu item. It looks pretty ugly. It sits OK horizontally when outside of the forum though.
Edit - actually I see what's happening here - in the forum header you've attempted to comment out the 'learn' link, but FF is still displaying it. Hence the menu turns out wonky. But funnily enough, the menu in /forums now renders OK in IE, but it still dissappears when you go back to the site home.
Testing across browsers of any site you launch is a good idea...