And now ARM's bringing out 64-bit ARM cores...

http://www.anandtech.com/show/6420/arms-cortex-a57-and-cortex-a53-the-first-64bit-armv8-cpu-cores

Not immediately relevant to RISC OS (for that matter, not relevant to RISC OS for quite a while), and the cores appear to be 64-bitted versions of the A15 and A7, but something to think about. I don’t think AArch32 will be dropped, simply because of how deeply entrenched ARMv7-A code is (iOS apps, and many Android games).

And, this isn’t the first ARMv8 design, AppliedMicro has been showing their X-Gene (which, as I understand, was actually the original basis for ARMv8) for a while now.

If it’s 64 bit OS you can run 64 and 32 bit applications, or you can just run a 32bit OS.
64bit RISC OS that supports multicore?

And the microkernel approach that Jeffrey Lee was discussing in the multicore support thread would be excellent for that – make a 64-bit microkernel, and the userland doesn’t have to care.