As an iPhone user, and app user, I can see the attraction in a dedicated app. However, designing for iPhone only is a bit like designing only for Internet Explorer in my opinion. That's generally consider poor form.
I do a bit of mobile web development and I personally think that it's better to design for mobile devices in general (such as to the dotMobi or MobileOK standards) and then use content adaptation to determine device capabilities (via Device Atlas, Wurfl or TeraWurfl) and render the site and images to the requirements of the device. dotMobi compliant sites should then work on any device, including the iPhone, so they're much more future proof.
If you went for a standalone iPhone app, you'd also need direct access to the database, which I don't think you'd get easily. If you produce a CSS layout for the vBulletin plugin I doubt whether OCuk would have too many issues about installing it.