An acquaintance of mine used this mod for vBulletin to make the forum easily accesible for mobile devices. It detects whether the device is a mobile one by looking at which webbrowser is used, and if it's a mobile device it loads a different theme. There are mobile themes listed at the bottom of the page.

It does use the device's own browser, but it's easy to apply and very effective. Also, this is platform-independent: You don't have to create apps for every major platform.