A few years ago the Xen developers sent a massive patch to the Linux kernel developers and it got rejected outright because it got far too intimate with way more of the kernel internals than it should have.

The other blog post linked from the OP's link mentions this: http://blogs.oracle.com/wim/entry/linux_mainline_contains_al...

Here's one of the LKML threads where the Xen guys sent a pull request straight to Linus, got rejected, and there was a bit of a shitstorm: http://thread.gmane.org/gmane.comp.emulators.xen.devel/64085

Things seem to have changed now, though -- it looks like the Xen developers have cleaned up their code and the Linux devs were willing to merge it into mainline.