IP.Board used to have an app. It still exists in the European iTunes App Store. However, it's not supported. IP.Board decided that the mobile web interface was sufficient, so they stopped development and support altogether.
As Landlord noted, an app wouldn't provide much more functionality than the mobile skin. About all I could think of would be notifications. IP.Board actually has (or had, anyway) a hook for that functionality, but I'm unsure whether it still is "active". And if it isn't, that would mean a lot of work for little return. Sorry.