Remember how Apple had x86 OS X being developed alongside PPC OS X? Wouldn't surprise me one bit that Apple has the same going on with ARM
If this rumour is true, then it wouldn't be a suprise at all. Although given the lack of development OSX has seen in recent years I would imagine the process is in a very early stage.
I would hazard a guess they would start with iOS and then build in the features from OSX.
The transition from PPC to Intel was not smooth, another change in platform would be a major pain in the ****. Especially for all the major software providers.