Some marketing experts say apps for iPhone and other mobile devices are the next big thing in small-business promotion, and will become as ubiquitous as the Web site. But like a Web site, poor delivery can waste all your good intentions. Here are some keys to creating a small-business mobile app that can help build your business -- and won't just suck your time: 1. Make it truly useful, not just a novelty item. 2. Beware the potential pitfalls. 3. Promotion, promotion, promotion.