From the Associated Press:

Apple is adding the option to enroll in a national donor registry by clicking a button within the iPhone’s Health app, which can be used manage a variety of health and fitness data. The software will come to all U.S.-based iPhones when the company updates its mobile operating system this fall.

And:

CEO Tim Cook says he hopes the new software will help ease a longstanding donor shortage. He told The Associated Press that the problem hit home when Apple co-founder Steve Jobs endured an “excruciating” wait for a liver transplant in 2009.

Update: Here’s a link to Apple’s official press release.