There's a multitude of applications on the iPhone for accessing Wikipedia, and they've just been joined by an official one from the site's creators - the Wikimedia Foundation.
It allows users to access articles formatted specifically for the iPhone, which is something that's already accomplished by the Wikipedia mobile website, as well as all the other Wikipedia apps around. The apps also offer advanced featured like the ability to search in pages, or save certain articles for offline reading.
The official app doesn't have any of those functions, though it will let you track your browsing history, so that you can see the Wikipedia effect - where viewing one article leads to hours of fascinated clicking in a variety of disparate subjects - for yourself.
The application is free, open source, and needs a bit of work, but if future versions integrate the best bits of the unofficial apps then it could prove to be a useful tool for settling pub arguments.