There are solutions out there that offer the promise of "write once, deploy anywhere" - in other words, you build a single version of your app that can be used on both iPhone and Android (and Windows Phone, Blackberry, etc. etc.). I've even used some of these tools myself on other projects. Sadly, in reality they tend to offer a significantly worse user experience on all platforms than a truly native app would. They can be slow and laggy, and fail to truly fit into that platform's style. Facebook began with an approach like this (building their mobile app in HTML5) but decided to throw it all out and start again with native apps because they just couldn't make that approach work well enough.