I also hope that one of the project goals for a MorphOS 4 would be to explicitly work towards making the OS (and SDK) more easily portable or even fully ISA independent altogether.

"we'll look into an architecture change, either X64 or ARM, whatever is interesting when we get there. [...] Meanwhile, we get into the 64 bit compatibility to prepare the ground. We will make it run in a virtual machine [...]. That's for development [...] so that we can start working on it without having a machine available yet."

http://www.amiga.org/forums/showthread.php?t=59765

Hmm... Develop now, choose architecture sometime later...