I have not had the pleasure of building a OS X kernel myself, but I have checked out the operating system since reading this thread and I have to say I am as excited as Neo about this - if Apple take the correct approach and open the source to allow people from all over to modify and scale the operating system, this could be massive. However that is a big if - it has it's advantages and disadvantages to Apple, we're just going to have to wait and see.
On the scalability issue still, OS X Server seems to me to provide a fair amount, I think the figures were something like over 1,000 simultaneous user connections, 4,000 open files per process, and there is support for multiple 100Mbit NIC's - sounds like it is moving in the correct direction. However I don't think it's a good route for Apple to go down if it involved releasing a desktop OS X and a Server OS X, it sounds too much like Windows, and we all know that the scalability in that operating system is an addon, rather than intrinsic to the actual kernel as it is, and always has been, with UNIX.
Sounds like Neo is right - we have to hope Apple release the source. Other than that I'm off to look at prices of Mac's, this has got me more than a little interested
Regards.