Imec, an electronics research institute out of Leuven, Belgium, has teamed up with the R&D Holst Centre and TASS, a software company out of Augusta, Kansas, to create a mobile and wearable ECG system based around an Android smartphone. It looks like the major achievement was overcoming the power hungry nature of Bluetooth, which was achieved by using a totally different, low power transmission system. Relying on less power should allow for longer continuous monitoring sessions while the patient is going about his day.
From the press release:

The interface is based on a standard Secure Digital Input Output (SDIO) interface on Android mobile phones, enabling the integration of all the features available on Google’s operating system (SMS, e-mail and data transmission over the internet, GPS to track user location). Moreover, the mobile phone’s hardware is extended to operate with low-power communication protocols and low-power radios, enabling long-term medical telemonitoring. As the interface is based on the Linux kernel, the system is also easily portable on other Linux-based devices, such as PDA’s or laptops. And, the system allows configuration of thresholds on the measured parameters and automatic sending of alerts such as SMS messages and e-mails based on these values.

Press release: http://www2.imec.be/be_en/press/imec-news/wirelesshealthnecklaceinterface.html
Reference: http://www.medgadget.com/archives/2010/10/continuous_ecg_monitoring_on_an_android_phone.html