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Jo is a JavaScript framework for mobile applications. It's fully compatible with PhoneGap, which it relies on for creating native applications. It's designed to be as lightweight as possible - its minified & gzipped JavaScript is just over 12KB with no dependencies. In addition to mobile Web apps and native apps, Jo can be used to build dashboard widgets.

Joe was built by Dave Balmer, a veteran Yahoo developer now working for HP on WebOS.

More and more web developers are earning their mobile wings. Mountain View-based Appcelerator just announced their beta release of Titanium Mobile - an open source, open standards tool that lets developers build applications with native UI elements, code compilation, device storage and geo-location APIs on both the iPhone and Android.

With Appcelerator's new mobile offerings, web programmers can avoid the pitfalls of Objective-C or Java, and use basic Javascript, HTML, and CSS to build their apps.

Portland, Oregon based startup Urban Airship demonstrated a beta service at today's Apple developers conference that will handle the heavy lifting for small iPhone app shops seeking to utilize the new push notification functionality and the ability to sell goods in-app instead of sending customers to Apple's store.

The service uses RESTful APIs and Open Source code libraries to handle "the tedious, annoying, difficult and troublesome parts of the development process" and offer outsourced scalability solutions.

PhoneGap is a very interesting development platform for mobile applications that lets developers build apps that work for multiple devices, including the iPhone, using only HTML and Javascript. That means far more people are able to develop mobile applications.

Recently, though, Apple has been rejecting an unusual number of apps built with PhoneGap from its app store. The company's reasons don't seem clear and developers want to know what's going on. We discuss three possible explanations below, but ultimately the problem appears to come down to the tight control that Apple maintains over the app store and iPhone.