Please see recent XBox port discussion. It appears that fencing on PowerPC is much trickier than we first thought. My recommendation is:
Thebest public document I've seen on the PowerPC architecture's memory modelis: http://www.ibm.com/developerworks/systems/articles/powerpc.html, but it does not appear to be enough to justify the sequences in N2745r. If anyone knows better specifications of the PowerPC memory model, please post them.