ShaderX8 is already announced. Proposals are due by May 19th, 2009. Please send them to wolf at shaderx.com. An example proposal, writing guidelines and a FAQ can be downloaded from www.shaderx6.com/ShaderX6.zip. The schedule is available on http://www.shaderx8.com/.
Thanks to Eric Haines for reminding me to add this to this page :-)