Biomics, a Russian Academy's of Science Journal on Biochemistry and Genetics, published an article on Ion Torrents pH-sensor-based genome sequencing history and progress:
"Rothberg Sequencing (Potential For Semiconductor Sequencing)" by Vladimir V. Zubov, Institute of Theoretical and Experimental Biophysics of RAS, Russia (see pdf link for download)
The article goes through the Ion Torrent genome sequencing machine generations, and talks about the recent developments of the company's pH-sensor. The pH-pixel size has shrunk down to 1.25um now and continues shrinking, while resolution went up to 13MP. The price of full human genome sequencing is expected to drop to $1000 in 2014, in part due to the higher resolution pH-sensor chips: 660MP Ion PII and 1.2GP Ion PIII.