The Xserve has officially been discontinued by Apple and will no longer be sold after January, 2011. Mac OS X Server will still be available on Mac Mini and Mac Pro (which will be the only option for Metadata Controllers on Mac OS X). Apple has produced a transition guide, available here.
I do like the Mac Mini server, I just can’t seem to fit a fibre channel card into them. Before the Xserve, we used to buy Marathon rack mount kits for G3 and G4 systems. I would anticipate that a new business will spring up that takes the place of the Xserve, putting Apple logic boards and CPUs into a smaller U configuration, although if that were profitable perhaps Apple would still be doing it… Either way, we had a good run with Xserve (sniffs and queues up a sad song).
Remember, remember the 5th of December, the day the Xserve was killed!