Apparently, at least as of 10.2.0.1, Oracle RAC is officially supported on RHEL5, but they forgot few things:

• in oraparam.ini "redhat-5" should be added to the supported OS list. (Yeah, that's kinda funny)
• the LD_ASSUME_KERNEL setting should be removed from crs/bin/vipca & crs/bin/srvctl (and maybe other places). LD_ASSUME_KERNEL sets glibc to old threads methods (pthread) which don't seem to be supported anymore in RHEL5..
• vipca (Virtual IP manager) won't run unless you manually set the interfaces for the first time using the 'oifcfg setif' command.

Apparently it recognized CentOS5 as RHEL5, which is cool.