The Tech Report heard that the ARM cores that AMD will integrate into its Opteron CPUs will not be an own design, it will be a pre-fab 64-bit CPU core from ARM:
One of the questions that lingered after yesterday afternoon's press conference was the exact nature of the CPU cores to be used. ARM offers licensees two basic options: rights to use a pre-fab CPU core and build it into a chip, or rights to build your own CPU core that's compatible with the ARM ISA. Most familiar ARM-based devices these days use the first option, incorporating something like the Cortex A9 into a larger SoC. A few use the latter option, licensing the ISA and building their own CPU core, including Krait, Nvidia's secretive Project Denver, and apparently Apple's A6.
AMD's talk of bringing its expertise with 64-bit CPUs to the ARM ecosystem might have led you to think that it had taken the second path, licensing the ISA, but that's not the case. Instead, AMD is licensing a 64-bit CPU core from ARM and building it into a chip—which AMD calls an SoC, or system-on-a-chip—that's compatible with the server-oriented Freedom Fabric interconnect AMD acquired when it purchased SeaMicro. This fabric interconnect will most likely be incorporated into the silicon along with the ARM cores.