I don't know for sure with CentOS-5 and CentOS-6 ... but certainly if mariadb is the default in Fedora, it will eventually be the default in RHEL as well.
FYI MariaDB has been built for EPEL5 and EPEL6 being maintained by the Fedora maintainer so a CentOS built package may not be needed in future:
https://bugzilla.redhat.com/show_bug.cgi?id=875150
RHEL6.5 and RHEL7 should be very interesting ;)