Being someone who builds my own systems, I'm waiting for several things before my next major upgrade:
A) Cost reductions in Socket 939 AMD processors
B) A Socket 939 mainboard with PCIe support
I'm not waiting however for BTX support. Dodec, while my analogy wasn't perfect, it's still an architecture, just a CPU architecture rather than a motherboard architecture (it was the one I could think of off the top of my head at the time). And it is true that Intel has had proposed architectures fail to be widely adopted; most of these failures are not widely publicized, and they have had plenty of success, more than enough to make their failures seem small by comparison. I wasn't saying BTX would fail; just noting that not all Intel proposals have succeeded as industry standards.
BTX can, in theory help cool a processor some by making sure the CPU is the first item to be cooled by the coolest air in the case; that which has just entered through the front intake fan. With the proper setup of a case with a dedicated cooling channel and a mainboard that strictly conforms to BTX, it may also be possible to keep voltage regulators and the mainboard chipset cooler. With an enthusiast-built system though, one can keep a case plenty cool with a bit of forethought to design choices. It will take some time for mainboard manufacturers to adopt BTX, and if it costs more to develop motherboards of this design, it may take longer yet. I'm also betting that BTX is more likely to occur in OEM systems first, who would rather use bigger case fans in a cooling-channel design than a smaller, less reliable fan for a CPU heatsink. ATX was adopted relatively quickly for multiple reasons, including the need for a new layout to accommodate a large Slot 1 CPU, and a need for a more organized, standardized, and solidly mounted port setup. There aren't as many readily apparent reasons for a quick switch to BTX.
As for power supplies, BTX setups will likely use the very new 24 pin power supply configuration. Most 24-pin power supplies will adapt to 20-pin (many include the adapter), but I've not yet seen a 20 to 24 pin adapter. I myself just recently got a 20-pin 470w Enermax a month ago; it looks like they already came out with a 24pin model that will soon supplant it. Currently, the only mobos I know using the 24pin spec are the new LGA775 Pentium 4 boards.
P.S. NuGuy, sounds like this is your chance to go to a system you've built yourself!
Consider taking the chance if you haven't done so already. It can be pretty rewarding.