Ubuntu 14.04 LTS (Trusty Tahr) is still a long way from the official launch in April, and the developers are still trying to implement the latest packages. The newest NVIDIA driver is now available for download from the official repositories.
Ubuntu developers were not so quick in previous years in upgrading the video drivers found in the official repositories. This fact changed in the last year due to the rise of Steam for Linux. Canonical can no longer afford to not upgrade to the latest video driver as soon as possible.
The 331.38 NVIDIA driver was made available only a couple of weeks ago, and now Ubuntu 14.04 LTS users can upgrade to this version.
You don't have to do anything special, just perform the routine system update and you will get the latest NVIDIA driver.