Question: Why does VirtualBox from the Ubuntu repositories not support USB? 

(While the [official release](http://www.virtualbox.org/wiki/Linux_Downloads) does support it?)

Answer: 
The USB code in VirtualBox is not open-source.The ubuntu repository is the open version,so the open source edition of VirtualBox doesn't have USB function.It's easy enough to install the full one from virtualbox's website.
As an update: since version 4, they re-architected VirtualBox so that the standard distribution is fully open-source, and the proprietary features have been [moved off into an extension pack](https://www.virtualbox.org/manual/ch01.html#intro-installing). So you can install the Ubuntu package, and download the extension pack from Oracle.