I came across this too. The misnamed device is still perfectly usable. If you boot from something with an unconfigured or misconfigured initramfs (such as the install CD?) and need to chroot into a system, deactivating the array and reassembling it, even using --scan, should name it properly.
Configuring /etc/mdadm.conf and then setting MDADM_CONFIG in /etc/genkernel.conf (or supplying --mdadm-config) before creating your initramfs will name the device properly. If you are not using an initramfs then I'm not sure this will be a persistent issue.
