Is launchd still being actively developed?
If the last time Apple publicly posted sources to launchd was when Snow Leopard shipped, it may be that there were no critical bugs in launchd that were worth fixing in Snow Leopard software updates, so none of the post-Snow-Leopard updates to the Darwin sources would contain any launchd changes.

When Lion ships, you can expect to see a huge new drop of Darwin sources based on Lion.  You may find that a lot has been going on behind the scenes over the last 2 years. Or not. But given that Apple has said Lion will ship sometime this month, your question might be 2 weeks premature.