Apple's XNU is de facto monolithic in nature. It simply reuses OSF Mach primitives to build the BSD Unix layer next to it. Mach IPC is used primarily for service discovery these days, I think, what with launchd being responsible for being the bootstrap server, which maintains name-port bindings for Mach services to call into. This is a side effect of Mach port namespaces being internal to tasks, and hence not having an intrinsic arbiter. In contrast though, the Hurd decentralizes this activity by just assigning a bootstrap port for the client making the RPC call to rendezvous with the server or library in question. These days, OS X seldom uses Mach IPC directly, but rather via XPC.

Mach had a surprising resurgence in interest over NextBSD implementing a ton of compatibility layers and mangling Apple sources in a weird, misguided attempt to make FreeBSD the new OS X. We'll see how it turns out.