[HT Ryan Baily]
According to this research paper, its pretty easy.
Srdjan Capkun, an assistant professor of computer science in the system security group at ETH Zurich in Switzerland, who led the work, says he was inspired to investigate the security of keyless entry and start systems after buying a car that had one. Capkun and Aurélien Francillon and Boris Danev, both researchers in the same institution, examined 10 car models from the eight manufacturers. They were able to access all 10 and drive them away by intercepting and relaying signals from the cars to their wireless keys. While they could relay the signals from the key back to the car as well, usually they did not need to because the key transmits its signals up to around 100 meters. The attack works no matter what cryptography and protocols the key and car use to communicate with each other.
Normally, when a wireless key is within a few meters of the right car, it detects a low-powered signal that causes it to issue a command that opens the car enable the ignition. The researchers used a pair of antennas to transmit these signals from the car to the key when the key was farther away, tricking the car into opening without the ordinary authorization. One antenna needs to be very close to the car, and one needs to be within eight meters of the key.
The researchers came up with two versions of the attack. In one, they ran a cable from near the car to near the key and used it to transmit the signals. They conducted the other wirelessly. Francillon says that the materials for the wired attack cost about $50, and those for the wireless attack cost between $100 and $1,000, depending on the electronic components used.