5.1 is not a different distro than 5.2. If you update 5.1 it becomes 5.2. You don't go out and say "update to 5.2", you just yum update, and it becomes 5.2. Think of it in Windows terms as Centos 5 sp1 (service pack 1) or Centos 5 sp2. If you want to stay with 5.1 you no longer get updates.
are you speaking as an official representative of CentOS?