now think about this: along with the new BTX cases, you'll need a new BTX motherboard, and with a new motherboard you'll also need a new CPU to go with it, and guess what?
that CPU will most likely come from Intel
I think BTX cases are basically a marketing technique for Intel to sell more of their CPU's
Intel had been caught with their pants down when AMD released their 64-bit CPU's that can perform better than Intel's best CPU's, and I think Intel is a little desperate now as they are not going to come up with a new CPU soon - unless they make a rushed effort (in which case the CPU wouldn't be that great)
heat is Intel's reason for creating the BTX cases.
sure CPU's generate a lot of heat nowadays, but that doesn't mean your computer is going to blow up just because you don't move to a new case design
just look at it like this: it doesn't cost that much to get a heatsink and fan, but what is the cost of a new case, motherboard and CPU?
now I'm not totally against BTX, but I don't think it's going to give you enough advantage over ATX to make the move anytime soon