Comment 10 for bug 513131

The reason this likely has not been enabled is because it's dependent on a configuration option which is noted to be deprecated, ie see drivers/ide/Kconfig:

menuconfig IDE
        tristate "ATA/ATAPI/MFM/RLL support (DEPRECATED)"
        depends on HAVE_IDE
        depends on BLOCK
        ---help---
          If you say Y here, your kernel will be able to manage ATA/(E)IDE and
          ATAPI units. The most common cases are IDE hard drives and ATAPI
          CD-ROM drives.

          This subsystem is currently in maintenance mode with only bug fix
          changes applied. Users of ATA hardware are encouraged to migrate to
          the newer ATA subsystem ("Serial ATA (prod) and Parallel ATA
          (experimental) drivers") which is more actively maintained.

          To compile this driver as a module, choose M here: the
          module will be called ide-core.

          For further information, please read <file:Documentation/ide/ide.txt>.

          If unsure, say N.

if IDE

...

config BLK_DEV_CMD64X
        tristate "CMD64{3|6|8|9} chipset support"
        select IDE_TIMINGS
        select BLK_DEV_IDEDMA_PCI
        help
          Say Y here if you have an IDE controller which uses any of these
          chipsets: CMD643, CMD646, or CMD648.

That being said, it does appear that the current Oneiric kernel config still enables CONFIG_IDE=y for powerpc. Thus, for the time being I'd be willing to enable the BLK_DEV_CMD64X option for Oneiric (assuming it builds) per this bug request. However, please note that there is no guarantee going forward this option will remain enabled and could potentially disappear due to the deprecated nature. Give me a bit and I'll try to post some test kernels for you as well. Thanks.