Core OS, Core Services, Media, Cocoa Touch
Fine. Kinda silly and arbitrary past the kernel/user boundary, but what I found most hilarious is that "Threading" is listed in Core Services (along with "Address Book" and "SQLite"), instead of in Core OS (which has things like "Sockets" and "Power Management".
Is that why threading sucks so much on Macs and iTunes can't do more than 1 thing at a time well? Because it's all in userspace? :-)