If you mean forums in general, forums using SMF (Simple Machines Forums software) are very easy to browse and post on from your BlackBerry. It seems like the forum software automatically detects the mobile brower and serves the content to fit. For example this one: Simple Machines Community Forum - Index
Also my forum running on phpBB is fairly easy to browse and post to without any special modifications or wap set up for mobile browsers.
vBulletin forum software forums, like this one, don't load well on the BlackBerry unless they have a corresponding wap site.