The US Food and Drug Administration and Homeland Security issued warnings about cybersecurity vulnerabilities in St. Jude pacemakers. This CSO Online article - 465,000 Pacemakers Vulnerable to Hacking Need a Firmware Fix discusses the firmware update needed to close the security holes. It also reviews the history behind this story and references research done by Bishop Fox which verified that these medical devices were exploitable.
This research also appeared in the eSecurity Planet article - When Risk Management Means Life or Death: FDA Approves Recall of 465,000 RF-Enabled Pacemakers and the Yahoo! News article - Nearly Half a Million Peoples' Pacemakers Are at Risk of Being Hacked.