This afternoon I got my grubby, little hands on x86_64 ISOs for CentOS 6. Since I lost at MUNI-roulette on the way home this evening, I took the opportunity to try installing them into a VM. At first glance, there is no "mock" package, which is rather troubling, but I am holding out hope that will be available when I reach connectivity.
If it's not, I'll have to spend the time to download the i686 ISOs to complete a Passenger build cycle. It would be a royal pain to do it that way, But I'm itching to get it out the door, so we'll wait and see.
Update: Very interesting: the mirror from which I grabbed my ISOs no longer has a 6 tree, so I guess this will have to wait until tomorrow or whenever they finally open the floodgates.