Red Hat Enterprise Linux has been out since November of 2010. Here we are 16 months later and it's not certified for database use by Oracle yet.
Yes, I know the database can be installed on RHEL6. In fact, I have it running in my development environment right now with no issues related to the OS/DB integration. Try to get Oracle Support to help you on those platforms for an issue unrelated to the OS and they still tell you they can't help you because the OS isn't certified.
Red Hat has submitted the results of their testing to Oracle almost a year ago!
Granted, Tikanga is fine for most stuff. But when the rest of your organization has moved on to Santiago, it's hard to justify why the 5% of Oracle machines need to be on the old version of the OS.
Some interesting posts on the topic by Tim Hall:
What if Oracle 11gR2 never gets certified on RHEL6?
Oracle Database 11gR2 on RHEL6: Certified or Not?