Unlike rovers sent before it, Curiosity is powered by a plutonium heat source. Tiny pellets of the radioactive material encased inside a radioisotope thermoelectric generator (RTG) provides an uninterrupted flow of electricity to the rover's instrumentation.
Previous rovers, like the currently operational Mars Exploration Rover Opportunity, used solar panels to harvest sunlight for energy, but this form of electricity production is at the mercy of the day/night cycle, dust storms and dust deposits.
Although Curiosity's planned mission lifespan is 2 years, the RTG energy source could extend its lifespan by 20 years. In that case, could the first manned mission land on Mars while Curiosity is still operational?
McCuistion said that although he could imagine astronauts walking up to Curiosity, the rover's instrumentation would likely break down before the energy source ran out - if humans did approach the robot, she'd likely be long dead.
For me, although it seems poetic to "bring the rover back" (or "leave no robot behind!") I'd prefer to leave Curiosity on Mars as a monument to the science she did in the "pioneering days" of Mars exploration.