On both Windows Phone and iPhone, the main constraints on resolution are the amount of onboard RAM as well as the speed that full resolution images are written to storage.
Here's a Twitter conversation where a few people asked the Photosynth team about this subject: http://bit.ly/psmpr
The real limiter is Photosynth's mobile pano app's shooting mode. Notice the following:
:: The video feed is constantly 'on' (never shutting down momentarily while a high quality image is written to storage like the default camera apps)
:: The shooting mode operates continuous 3D graphics
:: The shooting mode holds all captured shots so far in RAM until stitching begins.
:: The shooting mode continuously performs realtime image feature extraction and matching against previously captured shots.
That means that the one feature that makes Photosynth's mobile pano app so amazing (the shooting mode) is also what limits its resolution abilities until phone hardware improves.