Steve Jones - Editor (1/16/2008)
Sorry, the initial documentation said data center only, but since the question was written (a few months ago), it's changed. Or been updated.
Points awarded back and the question changed.
Unless I misunderstand the documentation, the "corrected" question is still incorrect.
Hot Add CPU is supported on
* the 64-bit edition of Windows Server 2008 Datacenter (first answer)
* the Windows Server 2008 Enterprise Edition for Itanium-Based Systems (not listed)
It is not supported on
* The 32-bit edition of Windows Server 2008 Datacenter (part of the second and third answer)
* The 64-bit edition of Windows Server 2008 Enterprise Edition (fourth answer).
Unless noted otherwise, "64-bit" is usually assumed to refer to x64, not to Itanium.