Back on topic.
id is one game maker well known for using Assembly coded routines - thus CPU specifc - for squeezing out every last drop of performance out of their code. But generally speaking it's not very common for game makers to resort to such techniques. It's somewhat safe to say that high profile games use C++ as the preferred language. C++ can be easily ported.
The biggest concern as far as porting is the simple fact that x86 CPUs are little endian and PPC CPUs are big endian. Perfect example: the original MD2 model loading code (Quake2 engine) was endian specific. They had to patch that code when ported to Mac. The data files themselves don't need to be changed, but when they're loaded into memory, the bits have to be rotated in order for it to make any sense on the PPC platform.
Xbox2 being PPC will at least eliminate the "oh did you remember to check for endian?" in the code. DirectX is just the graphics/audio/network layers. Those layers can be abstracted and replaced (think lego blocks). The bulk of the code that goes into a game is other things such as physics, AI, a scripting engine, and generally speaking the logic of the game. If all that is coded in clean C++ then it should be very easy to port.
I'm convinced that the lack of games ported to MacOSX is because of business politics, not technical hurdles. Who exactly pays for MacSoft or Aspyr etc. to port those games? Does the original publish pay or does MacSoft/Aspyr/etc have to pay/license the game? Either way, money is involved and some companies just aren't committed enough to shell out the $$ to port all these games.
Believe me, given the source code, anything can be done. There is nothing technical that stops developers from porting anything. It just takes time and money. So obviously it's the time/money factor that's holding back the ports.
And I also think that Xbox2 being PPC based will NOT affect the ease of portability.
