The release announcement for CentOS 5.5 can be read on their mailing list. There is also the CentOS 5.5 release notes.
The CentOS 5.5 packages and features match that of Red Hat Enterprise Linux 5.5. Among these changes include new hardware support, run-time memory allocation for KVM guests, EXT4 file-system support, Fibre Channel over Ethernet capabilities, eCryptfs support, and many other new items to assist those running CentOS/RHEL in a server or workstation environment.
Meanwhile, Red Hat has been focusing on delivering Red Hat Enterprise Linux 6.0 with there already being the first public beta.