NVIDIA Already Publishes An OpenGL 4.5 Linux Driver
The NVIDIA 340.23.01 Linux driver is available today and provides beta support for OpenGL 4.5 and the OpenGL Shading Language (GLSL) 4.50 versions. For tapping all of the potential of OpenGL 4.5, a Fermi, Kepler, or Maxwell GPU is needed. This is conveniently the GeForce 400 series and newer, which is only what's supported now after NVIDIA dropped pre-Fermi support from their mainline Linux driver.
The new NVIDIA 340.23.01 beta Linux driver with initial OpenGL 4.5 support is available for 32-bit and 64-bit architectures. NVIDIA also has out a beta OpenGL 4.5 driver for FreeBSD and Solaris too. These drivers can be fetched from this NVIDIA web-page. Take note that this initial NVIDIA beta driver is in the 340.xx series while NVIDIA just started the 343 beta driver series -- in time the OpenGL 4.5 support will be brought up to par with the mainline driver.