BTX technology isn't the successor to ATX anymore. It was conceived to cope with the massive heat problems that Prescotts were having, with the notion that future chips would be even hotter.
That is no longer true, with Intel changing roadmaps to produce cooler, lower-clocked chips that have better performance/watt characteristics.
Wikipedia has some links to articles treating BTX as a failed concept.
I don't think we're going to see the end of ATX for a long time, and I certainly don't think BTX will be the only form factor to suppass it.
Why do you need a BTX case? Do you already have a BTX factor motherboard?