Cross platform mobile development allows you to target multiple devices while being able to write minimal source code. This provides a couple of benefits. First, you have a lower barrier to entry, since typically HTML/CSS/JavaScript is easier to develop in than Objective-C or Java. Second, you reduce the number of required skills for your development team. Using cross platform development you only need to be proficient in one language/skillset, rather than Objective-C for iOS, Java for Android, Silverlight for Windows, etc. Finally, you reduce development costs and long term maintenance. Typically you just have one single codebase, which speeds up initial development. You also don’t need a large staff for each platform when it comes to long term maintenance.
And if you are involved with cross platform mobile application development, you know there is a continual debate with which platform to choose, Titanium or PhoneGap? Both promise native application development using tools front-end developers are familiar with: JavaScript, HTML, CSS. Recently I was able to use both on projects here at MarketNet, so I can give a you high-level comparison based on my experiences.
Titanium

Appcelerator's Titanium
The main selling point of Appcelerator’s Titanium is the fact that you are building a mobile application with a truly native user experience. From their website:
The Titanium Mobile SDK leverages over 5,000 device and mobile operating system APIs to create native applications that perform and behave just like they were written in Objective-C (iPhone and iPad) or Java (Android phone and tablets).
If you are comfortable with JavaScript, Titanium will be a breeze to pick-up, closely following the CommonJS mobile specification for structuring code. And with their newly released Titanium Studio, you have just one development environment to write all of your code. Titanium then translates this code and compiles it into native application code for both iOS and Android. The obvious benefit here is performance, native application elements will perform faster than using HTML, CSS, and JavaScript in a browser instance.
Another great feature of Titanium is the ability to extend native functionality with custom modules. They have an active Marketplace with dozens of modules for integrating PayPal, Dropbox, Urban Airship, In-App Purchases, and much more.
PhoneGap
PhoneGap is perfect if you are looking to embed an existing mobile site into a mobile application, with the ability to load it with extra features only available in native applications. Here you are using HTML and CSS to style and structure your app to display in a browser instance, not native components. From their website:
PhoneGap is an HTML5 app platform that allows you to author native applications with web technologies and get access to APIs and app stores.
One advantage is you can use one of the many mobile web frameworks out there, such as jQuery Mobile, to build a mobile site. Then just wrap it with PhoneGap, and voila, you can deploy to iOS, Android, Blackberry, and Windows Mobile.
The main disadvantage that I came across is that once your mobile site is complete, typically your integration has to be done directly in the underlying SDK’s IDE, such as Xcode for iOS or Eclipse for Android.
Summary
In short, Titanium utilizes JavaScript to compile directly to native code, giving you a truly native look and feel. PhoneGap gives you a native application wrapper for your mobile web site, and has a wide range of deployment options. Both have their purpose and are great ways to efficiently build cross-platform mobile applications.