First of - boxgrinder rocks!
Second of, I've just downloaded your RAW x86_64 appliance and have tried to build CentOS 6 EC2 ready appliance and - it worked! I just want to confirm with you that it's possible to build CentOS 6 appliance on Fedora 15 (I have read somewhere that it's not possible?).
Great! Just for anyone else picking up this thread, you *can indeed build Centos 6 with BoxGrinder*.
kaos monk; if you have found any documentation that says we can't build centos 6, please let me know where you found it and I'll update it.
edit: repost after kaos monk accidentally deleted the replies