The ECG Extension for HiJack is currently under development by Sonal Verma. The prototype shown here still uses a signal generator as ECG signal source. While the signal source has still about a 2x higher signal output compared to a real human ECG signal (~5mVpp) it shows the proper working of filters and amplification, as well as feasibility of integration.
The ECG board and HiJack interface are both fully powered by the iPhone headset port. The iPhone application can set the sampling rate by sending bytes through the left audio channel, and receives the sampled data from the microcontroller over the microphone port.
For more info, see http://eecs.umich.edu/~prabal/projects/hijack/