Offlining is different. On highend servers it's possible to power off cpus or put them into deep sleep states if just single cores.
Thanks for the link. About the paper, it's IMHO "nice research" but since they didn't have access to anything other than a dualcore laptop it is not highly helpful.
So far I have enabled the "powersave" cpu governor using xenpm.
At the Xen hackathon I also heard that the powersaving was at times broken for Xen kernels but from all I can tell this is working just fine on all versions that I have (Oracle 3.4 and AlpineLinux 4.1.2 with pvops)
You can also see / collect good statistics with xenpm, like with intel powertop on normal servers.
Oh and btw last weekend I did some large Nagios benchmarks. Doing a
service cpuspeed stop improved results by a good 10% :/
Anyway we can all do more on that topic so that we can hopefully some time live on the energy we get for free from our nature. And THEN we can have laaaarge datacenters