The Hydra.

TidBITS reports that Apple will allow virtualization of the Leopard version of OS X Server.

"Big deal. So what? Who cares?" you say. Well, this is the kind of thing that gets those guys who society rightly locks into server rooms all sweaty under their neck beards (neck beard jokes rule!). Now they can run multiple versions of Leopard Server on one box, alongside Windows and Linux. The Macalope, of course, knows several real Hydras (great dinner party guests -- all those heads and only one stomach), but this is the server equivalent.

Ben Rudolph of Parallels emphasized this final point, saying, "We're hearing from our customers - like you are from your readers - that the 'holy grail' of Xserves is to run multiple, isolated, near-native instances of Mac OS X Server on the same box, at the same time. If you couple that with the ability to run Windows and Linux next to those instances of Mac OS X, you've just made Xserves even more compelling for enterprises large and small, even non-traditional Apple shops."

[yipcanjo]

Who is using Apple servers?

You don't hear much about them. What are they doing? Web boxes? Workgroup servers? Is anything truly "production" being served up on Apple servers / OSX server??

[Orenge]

Large-scale Xserve deployments

I can answer that: Xserves have been used for widely-varied and VERY large-
scale applications. I have a list (the info's around a year old and was never
exhaustive--just a sampling) that I'll paste below, plus the iTunes Music
Store itself (and presumably the entire Apple Store too) runs on Xserves.

* 256 processors - US Army Space and Missile Defense Command's LIMIT
cluster
(128 dual G5 nodes, at Florida A&M U,
http://www.macnn.com/articles/05/11/29/u.s..army.projectused to develop
methods for detecting hazardous materials)

* Under construction - Omneta cluster
(Thousands of dual nodes planned,
http://www.eweek.com/article2/0,1895,1861506,00.asprented out for
corporate data processing in Europe)

* 260 processors - York NeuroImaging Centre's cluster
(130 dual nodes, largest G5 cluster in the UK,
http://www.medicalnewstoday.com/medicalnews.php?newsid=25160used
for brain research)

* AstroVision's cluster
(Xserves running Tiger, QuickTime 7 and Xgrid, will be the fastest
supercomputer in Australia,
http://australianit.news.com.au/articles/0,7204,12824308%5E15321%5E%5E
nbv%5E15306,00.htmlused for live satellite image processing starting in
2007)

* 448 processors - Bowie State's "Xseed"
(224 dual 2.0 nodes, http://www.macworld.co.uk/news/index.cfm?
RSS&NewsID=11043used for animation and other computation)

* 1280 processors - U. of Illinois' "Turing Cluster"
(640 dual 2.0 nodes, possibly to be doubled,
http://www.cse.uiuc.edu/turing/used for a range of academic research and
replacing a Dell/HP Linux cluster)

* 250 processors - U. Pitt's Human Genetics cluster
(125 dual G5 nodes http://www.apple.com/science/profiles/upittused for
genetics research)

* 200 processors - GeoCenter cluster
(100 dual 2.0 nodes
http://www.geocenter.com/news/xserve_overview.htmlused for seismic data
processing)

* 1344 processors - French CGG cluster
(672 dual nodes, http://www.hardmac.com/niouzcontenu.php?date=2004-
09-03#2783integrated into an existing 40 TFLOP cluster for oil prospecting)

* 48 processors - Louisiana State's "Nemaux"
(24 dual G5 nodes with Xgrid, http://www.tgc.com/breaking/2344.htmlused
for 3D animation, audio, and scientific computing)

* 2200 processors - VA Tech's "System X" aka "Big Mac"
(1100 dual 2.3 nodes and counting, http://www.tcf.vt.edu/used for a range
of academic research - with
http://www.businessweek.com/technology/content/aug2004/tc2004083_71
26_tc153.htmmuch larger Mac clusters System L and System C under
consideration)

* 256 processors - UCLA's "Dawson"
(128 dual 2.0 nodes, http://www.macworld.co.uk/news/index.cfm?
home&NewsID=10601used for plasma physics research)

* 64 processors - Australian Defence Force's "Checkmate"
(32 dual nodes running Tiger, half G5 and half G4,
http://www.smh.com.au/news/Next/Apple-upgrade-bears-
fruit/2005/05/02/1114886287215.htmlused for Chess-based command
and control simulations)

* 86 processors - UNC's cluster
(43 dual nodes, http://www.apple.com/pro/science/giddings/used for
proteomics research)

* 76 processors - UC Davis's cluster
(38 dual nodes, http://www.apple.com/science/profiles/benham/used for
Genome Center research)

* 72 processors - UC Santa Cruz's cluster
(36 dual nodes and counting, http://www.macworld.co.uk/news/index.cfm?
NewsID=9247used for a range of academic research)

* 3132 processors - US Army's "MACH 5"
(1566 dual nodes, http://www.macworld.co.uk/news/index.cfm?
NewsID=8971used by the Army and NASA for hypersonic flight research)

* 512 processors - U. Maine's "Baby MACH 5"
(256 dual nodes,
http://www.umaine.edu/news/071904/ArmySupercomputer.htmused for
software development and optimization for MACH 5)

* 52 processors ? UC Irvine's "HIPerWall"
(50 30" Cinema Displays and 26 PowerMac G5s
http://www.apple.com/science/profiles/hiperwalldelivering over 200
megapixels--double the previous record--of resolution for scientific
imaging)

* French-speaking Belgian schools
(http://www.hardmac.com/news/2007-03-20/#65533300 Intel Xserves plus
additional Macs)

* DTS Digital Images
(600 PowerMac G5s and 700 TB storage,
http://www.mi6.co.uk/news/index.php?itemid=3780used for digital film
restoration)

* US Navy
(Xserves on submarines
http://www.computerworld.com/softwaretopics/os/macos/story/0,10801,83
783,00.htmlused for their Linux-based sonar imaging system)

* Polish news channel TVN 24
(30 Xserves G5s, 50 PowerMac G5s, 22 Xserve RAIDs, 55 Xsans, 10
PowerBooks, 50 Final Cut Pros etc., and growing,
http://www.apple.com/uk/pro/video/tvn/index.htmlused for server-based
video editing)

[Mr. Blister]

cheaper OS X Server hosting perhaps?

Could this mean cheaper (virtual) OS X Server hosting? I'd rather see OS X than
Linux running my webapp... I'm always a little nervous with Linux that I'll bork
something up ...

[The Real John Muir]

How about on generic boxes?

But does the licence allow for virtualisation on non-Apple hardware?

This is a big question simply because if the answer is yes (which I doubt) it means there's a legitimate market for the inverse of Parallels Desktop for Mac ? a Mac virtualiser for Windows. Eerie! And quite possibly a big mistake for Apple whose OS is their crown jewels.

[Orenge]

No generic boxes

That's clear from the linked article:

"This License allows you to install and use one copy of the Mac OS X Server
software (the "Mac OS X Server Software") on a single Apple-labeled computer.
You may also install and use other copies of Mac OS X Server Software on the
same Apple-labeled computer, provided that you acquire an individual and valid
license from Apple for each of these other copies of Mac OS X Server Software."