Also, all you need is one insider kernel developer to get all the source code anyways. I always find these kinds of initiatives silly -- A lot of companies think that an insider is a side-channel attack when really it's the main vector.
OTOH, there does seem to be a fair amount of competence where it matters though. In the couple companies I worked for the private keys used for signing things were very quietly kept hidden from the vast majority of the engineering teams/etc. AKA, it was possible to create an development/test builds all day long, but creating valid license keys/firmware updates/etc for the builds given to customers was limited to a formal process which contained the keys. The private keys were only available to a couple people tasked with maintaining the automation from which the builds/keys/etc came from. Those people rarely had a need to move/etc them either, and such activities were done in the open.
You mean this source code? http://opensource.apple.com/source/xnu/