Qt 5.3 as the latest approximate six-month update to the Qt5 platform adds a new QtQuickWidget for better integration between Qt Widgets / Qt Quick, a new QtWebSocket module for Web Socket protocol support, improved printing support, major improvements to Android / iOS support, support for Windows Runtime platforms, new target binaries, and Qt Creator 3.1 Beta is included with the installers. While there's new features, including new platform support, the overall theme of Qt 5.3 is stability and performance improvements.
The Qt Android improvements and work on iOS support consist of better input method support on iOS, better positioning support for both platforms, Bluetooth support on Android, and many other minor enhancements.
The new Microsoft Windows Runtime platforms are for Windows 8/RT and Windows Phone 8, with the new target binaries being Qt for Visual Studio 2013 and Qt for WinRT and Qt for Windows Phone. The Windows Runtime support in Qt 5.3 is considered beta quality with full support expected in Qt 5.4.
More details on today's Qt 5.3 Beta release can be found via this Digia blog post.
This beta release comes a few weeks after the Qt 5.3 Alpha while the official 5.3 release is a few weeks out -- most likely around the end of April.