This isn’t quite lights-out, and Xserve ran the same version of Mac OS X Server you could install on any other Mac.
I managed a bunch of XServes for a while, they were incredibly good hardware. The Mac Server software kinda sucked (not the LOM stuff, it was as good as any of the LOM from Dell, which is to say, not amazing, but workable).
Apple FINALLY lets you turn on your Mac remotely, without having to press the power button.
To you and me, that sentence needs the word "again" appended to it.