According to a prepared statement, Serlet says he wants to "focus less on products and more on science." Serlet has been at Apple since 1997. Before Apple, Serlet worked with Steve Jobs at NeXT and played a key role in developing NeXTSTEP, the predecessor for Mac OS X. Serlet has overseen the releases of Mac OS X Tiger, Leopard and Snow Leopard.
Serlet's successor is Craig Federighi. Federighi is currently the vice president of Mac Software Engineering and will now report directly to Steve Jobs. In a statement, Serlet says, "Craig has done a great job managing the Mac OS team for the past two years, Lion is a great release and the transition should be seamless."
As John Gruber notes, it's difficult to overstate the influence Serlet has had on the modern Mac OS ecosystem. Serlet has literally been helping shape Mac OS X since the beginning -- from overseeing the OS structure to helping define the Cocoa libraries.
That work has evolved from powering high-end workstations (NeXTcubes and others) to desktops and laptops and now devices such as the iPhone, iPad and Apple TV.