BC is important feature to a lot of people and not important to a lot of others. Being a fan of clean installations and fewer cables, I prefer backward compatibility. Why have three Playstations, when one will play games from all three generations with potential graphical/aural improvements for prior generation games?
The one thing that is most definitely not a hurdle is the x86 --> PPC architecture. Microsoft completely absorbed Connectix some time ago, the maker of the popular VirtualPC Windows emulation software for Apple PCs. Aside from that fact, Microsoft is producing its next operating system with the evolution of the computing landscape in mind. Plenty of reputable sources claim that Windows Longhorn (codename) will come in forms compatible with current and future single x86 CPUs, multiple x86 CPUs, 64-bit CPUs, PowerPC CPUs, and potentially even compatible with future Cell technology. Scalability is huge with the next Windows.
Also, Microsoft didn't just buy three PPC CPUs from IBM and throw them in the Xbox360 (like the near-stock Intel Celerons in the Xbox), Microsoft Hardware and ATI were deeply involved in the CPU design this time around, it's core technology just happens to be PPC. Microsoft is the world's premeire software company. Considering that the OS integrated with Xbox360 is more than likely Longhorn-based, the x86 issue is null, really.
ATI was interviewed long ago stating that they would have no issues replicating the graphics protocols found on the nVidia hardware of the Xbox in their new chip. A couple weeks later nVidia stated publically that the rendering technology within the Xbox was not Microsoft's property and implied that Microsoft would have to pay royalties to them if they even wanted to emulate it. Pfft. We'll see what happens. Based on sporatic comments made by ATI and Microsoft, backward compatibility doesn't seem to be limited technically.
I'm hoping they do it, because I want it and they only stand to gain on the deal. Would you honestly find a customer that WON'T buy a new Xbox solely because it IS backward compatible?