The GL_ARB_viewport_array extension is required by the OpenGL 4.1 specification. This extension was first enabled by Intel Mesa developers and since then was also picked up by the Radeon Gallium3D code. The support a few months back was then added to the Nouveau NV50 Gallium3D driver but now is being worked through for the "NVC0" driver that provides the support for NVIDIA GeForce 400 "Fermi" graphics cards and newer.
Developers interested in learning more about this OpenGL extension can find the specification at OpenGL.org.
The ARB_viewport_array support for NVC0 is currently being worked through for the NVC0 Gallium3D driver via this patch-set that will hopefully be ready for mainline in the Mesa 10.3 release (or potentially what might be known as Mesa 11.0 pending GL 4.0 compliance) later this summer.