There are a few mobile theme options available to you (See dropdown bottom-left of each page), each with their own limitations.
A specific app is not something we've pursued for a number of reasons - time, cost and legal being the main barriers. Not opposed to it though.
The future of apps for forums is a little debatable. vB5 is likely to drop the development of it's mobile theme (which we currently use) and, instead, concentrate on responsive theme design that can adapt to screensize, essentially providing a full-featured front-end. But we'll see if that comes to fruition. vB have promised much over the years and not always delivered.
It's also not clear how it's mobile suite package (which includes a pre-developed app) will continue post vB5 beta. There's been conflicting messages about it's future, and it's not clear what the benefits really are over and above a decent mobile-specific theme.
Although... frankly, we haven't really considered how we'll respond to vB ceasing feature development of vB4 or what our upgrade plans are (if any).
As a compromise (if people really want additional mobile options), we could revisit the ForumRunner / Tapatalk options again. We recently discussed this and felt that it might be worth looking at again. However, I took a look at ForumRunner about a month ago and, frankly, thought it was dire. Haven't used Tapatalk.
