That "good faith effort" is required for all packages (GPL or not).
And yes, it is legally necessary make that good faith effort not to infringe upon someone else's trademarks.
This is specifically called out here: http://www.redhat.com/about/companyprofile/trademark/
I've just read that document and it seems to say that you could take all of the RPMs exactly as they are built by RedHat and include them on a disk, and you can label the disk "Centos 6", and you are completely within the guidelines.
PJ