As usual there are the release notes and the technical notes in two separate documents. The focus of this release has again been bug fixing. So a lot of fixes and minor enhancements especially in the s390utils have been delivered.
The IBM documentation for RHEL 6.3 for System z is on developerworks.
If you are experiencing problems with accounting CPU time on RHEL 6.3 on System z, please upgrade to kernel 2.6.32-279.5.1.el6.s390x. This contains a very important fix for System z.