I have made a thread about this before: viewtopic.php?f=120&t=13158
I have also used the HMD! It has a lot of issues, some of which they outline in the paper. It is almost impossible to get the phones to render in sync, and even if you could, you are limited to custom content that you create specifically for the HMD. Besides... The resolution is actually lower than that of the PR3.
Still really cool, though! If I were to make a smartphone based HMD, I would buy the upcoming Samsung Note (5.3" 720p OLED display), and use it with optics similar to the PR3.