"PCM uses heat generated by an electrical current"
What level of heat? Will there be any problems implementing PCM as a Package-On-Package where the RAM/Storage package sits on top of the (usually heat-generating) ARM SoC to save board space? If it can't be implemented as POP, PCM may turn out to be less optimal than current technologies in terms of board design. However, given it's performance characteristics that may not be a significant issue.