NASA uses their deep space network which is essentially an array of antennas located on different areas of the world to communicate with satellites and other devices in space. As far as I can tell there aren't any relay type devices out in space that amplify or repeat the signal.
Found another article from jpl:
http://mars.jpl.nasa.gov/msl/mission...communication/Not only will the rover send messages directly to the DSN stations, but it will also be able to uplink information to other spacecraft orbiting Mars, utilizing mainly the Mars Reconnaissance Orbiter and Mars Odyssey (if necessary) spacecraft as messengers that can pass along news to Earth for the rover. The respective spacecraft will mainly "talk" via their UHF antennas. The Mars Reconnaissance Orbiter carries an Electra UHF payload that could potentially be very valuable in navigating the Mars Science Laboratory safely toward Mars. The Ka-Band package aboard the Mars Reconnaissance Orbiter will serve as another possible pipeline to "talk" to the Mars Science Laboratory (read more about the Mars Reconnaissance Orbiter Engineering Instruments).
The benefits of using the orbiting spacecraft are that the orbiters are closer to the rover than the DSN antennas on Earth and the orbiters have Earth in their field of view for much longer time periods than the rover on the ground.
Because the orbiters will only be between 160 and 250 miles (257 and 400 kilometers) above the surface of Mars, the rover won't have to "yell" as loudly (or use as much energy to send a message) to the orbiters as it will to the antennas on Earth.
Looks like the talking to orbiting spacecraft is not to increase the speed of communications but to increase the ability of earth and the rover to talk to eachother over longer periods as the orbiting satellite will be in Earth's view for longer periods of time.
Also, it looks like the orbiting satellite is only around 160-250 miles above the orbiting satellite so it doesn't have to use as much power to talk... the satellite can do that for it and power can be reserved probably for moving around or performing experiments instead of communication.