Five start-ups unveiled services for building mobile applications and Web services this week at DEMO Spring 2011. MobileNation, Bizness Apps, Cloud9 and Ajax are among the start-up companies showing off their services. Cloud9 is a platform-as-a-service focusing both on mobile apps as well as Web tools that run on desktop Web browsers. The company’s “Small Business iPhone and Android Apps” service helps customers build apps in 20 minutes and manage them for $40 a month.”]