While based upon CentOS, the 6.1 release of Rocks isn't pulling in CentOS 6.1 but rather the latest 6.3 packages where RHEL/CentOS is at today. Rocks 6.1 is codenamed "Emerald Boa" and is being made available for i386 and x86_64 architectures. A Rocks 5.6 update is also being planned for when CentOS 5.9 is released.
The big addition to Rocks 6.1 is support for ZFS on Linux. The ZFS file-system is being supported by this UC San Diego sponsored Linux distribution through the use of LLNL's ZFS on Linux project. While ZFS is supported as a kernel module (not via FUSE), ZFS as a root file-system isn't yet supported.
Other changes include using host-based SSH authentication by default, two-factor SSH authentication using Google Authenticator Apps for Android/iPhone, easy support for generating QR codes, a new kernel-org roll for easily using vanilla Linux kernels from Kernel.org, and added Infiniband support.
There's also other changes to Rocks 6.1 as noted in the release notes. The release announcement is at RocksClusters.org.