Nowadays, most computers have several processor cores. However, most computer programs are still designed to only use one processing unit. As a convenient and portable way of writing software using all the available processing power, Qt provides the excellent QtConcurrent framework.
In 2010, a Google Summer of Code project examined the suitabilty of using Qt concurrent for rendering the map image in QGIS using several processor cores. Following that approach, each layer renders its image in a separate thread. Once all layer threads are finished, the layer images are composited into one map image and the labels are drawn on top of it. Despite providing good results, that code was unfortunately never merged into the main development branch.
QGIS Enterprise 13.05 will provide the capability of multithreaded rendering. A screencast of the new functionality shows that not only the render time is shorter using multiple cores. More important is to have the possibility to cancel the render progress and the labeling any time, thus achieving a much more responsive user interface when navigating maps.