]
] 64-bit extension technology is one of a number of
] innovations being added to Intel's IA-32
] Server/Workstation platforms in 2004. It represents a
] natural addition to Intel's IA-32 architecture, allowing
] platforms to access larger amounts of memory . Processors
] with 64-bit extension technology will support 64-bit
] extended operating systems from Microsoft, Red Hat and
] SuSE. Processors running in legacy* mode remain fully
] compatible with today's existing 32-bit applications and
] operating systems.

Here's Intel's AMD64 clone. You have to dig a bit for them to come
out and say that its AMD64 compatible ... but then at this point,
they really can't afford for it not to be. Itanium issues notwithstanding, Opteron would eat Intel alive in the low-end server market if they didn't have an offering.

Intel Corporation - 64-bit Extension Technology