My 64-bit capable system is only listed as certified for X86; is it considered supported if running under a 64-bit Red Hat operating system?

In regards to support on a particular system, the level of support is defined by the status of the system on the Red Hat hardware compatibility list (HCL). Systems not listed on the HCL are supported on a reasonable-efforts basis whereas certified hardware is supported to the extent of your SLA by Red Hat through it's relationships with the hardware vendor.

With the introduction of AMD64 and Intel® EM64T it becomes necessary to take special care to note the release the hardware was certified against. Not all of the hardware capable of 64-bit operation has been certified for use with the 64-bit versions of Red Hat operating systems. These systems will be considered certified only while running the appropriate 32-bit versions of Red Hat operating systems.