WebGL is derived from OpenGL ES 2.0 and WebGL implementations are already available within Mozilla Firefox 4.0, Google Chrome 9.0, an early Opera build, and the latest Apple Safari builds.
As part of this GDC WebGL launch, Khronos has also put out a WebGL Test Suite and a WebGL Reference Card.
The press release announcing the ratified WebGL 1.0 specification can be found at Khronos.org.
In terms of Linux support for WebGL, there are both open and closed-source drivers that support OpenGL ES 2.0. OpenGL ES 2.0 is supported by both the proprietary AMD and NVIDIA Linux drivers and in the open-source world there is fortunately already OpenGL ES 2.0 support within Mesa and Gallium3D. Though the open-source OpenGL ES 2.0 support has been a work in progress for some time and will require a very up-to-date graphics driver stack. Some browsers though, such as Mozilla Firefox 4.0, are blacklisting most Linux graphics drivers (including the open-source drivers). The best bet right now is the NVIDIA blob. By year's end there will hopefully be reputable Linux GPU driver support shipping "out of the box" that is white-listed in most distributions with a WebGL-enabled web-browser.