[Updated Aug 7 2006; see below.]
Tom Yager at InfoWorld points out that Apple has released the source code to everything in the x86 build of OSX Darwin except for the kernel, xnu.
He also makes what appears to be a completely and utterly unsubstantiated statement about why:
Thanks to pirates, or rather the fear of them, the Intel edition of Apple’s OS X is now a proprietary operating system.
First of all, what? That's a bold statement. Got any source for that claim? That seems like sheer FUD, deliberately sensationalized to create a stir and bring people to the site (and therefore sell ads). From everything I've seen, and I've worked with these people, Apple's security team and upper management know better than to rely on security through obscurity. And to date I don't know of any official or even unofficial statement from Apple about xnu-x86 -- just the fact that several people have noticed that the source still hasn't been released.
[Update 5/22 via John Gruber: Apple's Product Manager for Open Source reiterates that Apple has not made any announcement yet, and drops what sounds like a big hint that xnu-x86 will eventually be released.]
Let me offer two guesses at the real reason why we haven't seen source to xnu-x86 yet:
The xnu-x86 source might leak information about a future product. The obvious candidate is the only missing link in the x86 line, the pro desktops. You know, the ones that will replace the "PowerMac". Let's call them "Mac Pro" for lack of a better name.
For example, if the highest end Mac Pro desktop machines were planned to have, say, 4 CoreDuos packed into them for a whopping 8 CPU cores, then chances are you would see traces of that support show up in several parts of xnu. And when Apple releases a major rev of xnu, there are always some people who pore over it looking at the diffs.
Apple does have ways to keep prerelease stuff out of the source release, of course, but it adds a layer of complication and risk to go back and hack that up after the fact. Maybe this time they decided it was just simpler to drag their feet for a while until the entire new line has been announced.
The xnu-x86 source might currently contain a small amount of licensed proprietary code that does not belong to Apple. If that's the case, they simply might not be legally allowed to release it in its current form.
Maybe it's virtualization code from Intel, or some sort of Trusted Computing gobbledygook which is currently dormant. If they can't negotiate terms to release it, then they might have to factor the sourcebase somehow to link that other code in separately. Factoring that out seems like it would be totally possible, but kind of a messy task since it's at such a low level in the kernel and they can't sacrifice any performance to do so.
Personally I think #1 fits Apple's modus operandi perfectly. But #2 is also the kind of real-world consideration that could delay a source release for an unknown period of time while the lawyers work things out. I will grant that "fear of pirates" is another technically possible reason why we haven't seen the source, but then again the same could be said for "fear of ninjas".
We'll see what happens. Either way my gut feeling is that this is just a delay in the source release, not a permanent switch to a completely closed-source kernel.
Several changes were made in order to publish the kernel (xnu) sources. As a result, the kernel built from these sources differs from the one found in the 10.4.7 software update. In order to accommodate these changes, several kernel extensions were also modified and must be downloaded and installed in order to run a kernel built from these sources on Mac OS X 10.4.7 for Intel.
Based on that comment, it sounds like the answer is #2 and for the xnu-x86 release they moved some code from the kernel into a kext.]