From Fedora Project Wiki
I think this is a really bad idea. 64-bit in userspace buys you nothing for almost everything, and for those things it _does_ help, you can already install 64-bit versions.
You're talking about splitting the already relatively small PPC userbase into 32-bit and 64-bit parts, and doubling the amount of QA and release work we have to do for PowerPC. The 32-bit code is mature and well-tested already.
You'd do better finding any remaining multilib problems and fixing them, rather than changing wholesale to 64-bit.