NASA's newest Mars rover, Curiosity, is currently scheduled to arrive at Mars on August 6 of this year. There's a fourteen minute delay in communications and it takes seven minutes to go from orbit to the ground. By the time they get confirmation that it's starting to de-orbit, the rover will be on the ground and they won't know if it landed safely or not for another seven minutes.
Here's a quick video explaining the landing process, and how they've come up with a pretty amazing automated process in order to get the rover on the ground.