CentOS is (virtually) identical to the Redhat release of the same version number (main differences just being the removal of Redhat specific branding). We run all our installs on 5.5 x64 with no issues at all. Any Splunk documentation referencing "Redhat" can be applied directly to a CentOS install.
Splunk works well under CentOS x86_64. CentOS does use older versions of some packages, particularly Python. Not generally a big deal since Splunk includes its own copy of Python, but it can be annoying at times, e.g., when testing scripts and forgetting to point to Splunk's copy.