Every mobile platform has its own technology, learning curve and investment the developer need to make. Cross-platform frameworks ease the pain of "doing it multiple times" and provide more uniform environment
Mobile development platform for javascript developers: Appcelerator Titanium lets you use web technologies, open source and cloud computing to build native apps for iPhone, Android, Windows, Mac OSX and Linux.
The Only Open Source Mobile Framework That Supports 6 Platforms. PhoneGap is an HTML5 app platform that allows you to author native applications with web technologies and get access to APIs and app stores
Query Mobile: Touch-Optimized Web Framework for Smartphones & Tablets: A unified user interface system across all popular mobile device platforms, built on the rock-solid jQuery and jQuery UI foundation. Its lightweight code is built with progressive enhancement, and has a flexible, easily themeable design.
Calatrava is a cross-platform mobile framework that lets you share the core logic of your application across iOS, Android and Mobile Web, but unlike other XP toolkits it allows you to always write the highest quality native UI you need.