Hi Jun, Forumbee forums were designed from the ground up to work inside of mobile applications. We used responsive web design and tailor everything for small screens and touch devices. All of our UI supports retina and high density displays. Our goal is to be the best choice for forum software for mobile apps, so we are continually enhancing our mobile capabilities.
For iOS, you can use the UIWebView class to embed your forums (see Apple's iOS developer reference entry).
Give it a try, we'd love to hear your feedback.