I'm just your fellow user, guys, so don't take my questions as coming from the Photosynth team. Do other panorama stitching mobile apps do this? I'm thinking of AutoPano's and Occipital's 360 app as good examples.
To be honest, some mobile users have already found that the amount of RAM necessary to stitch panoramas comes fairly close to exceeding the amount of RAM that their device actually has ( http://photosynth.net/discussion.aspx?cat=a6bad539-de7d-4a16-8c4d-b143f7d5f984&dis=1b768606-ebb1-4d4d-b18c-a11979d746d8 ), so trying to actively run other apps while expecting a panorama to make progress stitching seems a bit naive to me.
As far as I know, your phone just isn't a powerful enough computer to multitask while stitching panoramas. You might say that phones will continue to get more RAM and faster CPUs, which is true, however I suspect that people would rather spend those resources on higher resolution than multitasking.