Sorry, I don't use CentOS 6 now so I cannot check, but:

Could it be that RHEL6 changed the default superblock mdadm is creating? CentOS 5 is creating a 0.9 superblock. Some "other OS" are creating 1.2 per default. But you can change that on the command line.

If that's the case it's not a bug but a feature..

GS