Windows driver version 280.28 and Linux drivers version 280.10.01.02 provide full support for OpenGL 4.2 and GLSL 4.20 on capable hardware. This driver also supports several new OpenGL extensions for both 4.2-capable GPUs and older GPUs. The driver download links are at the bottom of this page.
OpenGL 4.2 Driver Release Notes
You will need any ... Fermi based GPU to get access to the OpenGL 4.2 and GLSL 4.20 functionality
As it came unannounced, no GPU tool recognizes 4.2 yet properly.