I've always said that a real computer must be able to replace CPU/memory during operation. This has been possible for quite some time with a subset of the SPARC systems available from Sun and Fujitsu. Intel has this hardware feature available with their QuickPath architecture and AMD have had it since the introduction of HyperTransport 3.0.
Today Hot-plug support for CPU and memory for the Xeon 7500 platform was added to the OpenSolaris source.
It will certainly be of good use as X86 systems gets larger and more powerfull, one day they might even be used as mission critical enterprise servers. I sure hope that the current type of BIOS has been replaced by that day so that they can be managed directly over RS-232 or ethernet without the need for a graphical console. I know some X86 servers are managed over ethernet, but the still use graphics as the console, only sending it over the network.
PSARC/2009/104 Hot-Plug Support for ACPI-based Systems
6862510 provide support for cpu hot add on x86
HyperTransport 3.0 Press Release
Whitepaper Intel Nehalem
The heads-up is here:
"Introduces support for hot-adding cpus and memory to a running Xeon 7500
platform. See your OEM platform documentation to determine whether your
system supports this functionality."
Talks I have given
5 months ago