I came across this and it seems that ATX cases and mobo standards are becoming a thing of the past and BTX is going to be replacing it.
As is made evident by the proliferation of Small Form Factor (SFF) machines into the market, the need for a smaller, quieter successor to ATX has been building over the past 8 years. A couple of IDFs ago, Intel announced their development of the successor to ATX, which was codenamed Big Water. At the Fall 2003 Intel Developer Forum, Intel officially branded Big Water as the Balanced Technology eXtended form factor - or BTX for short
To read more about it http://www.anandtech.com/showdoc.html?i=1876