Copied from Wikipedia:
Some Americans are interested in Japanese domestic market vehicles like the Nissan Skyline. In 1999, a California company called Motorex had a number of Nissan Skyline R33 GTS25s crash-tested. They submitted their information to NHTSA and petitioned for 1990-1999 GT-Rs and GTSs to be declared eligible for import.[4] Many Skylines were subsequently imported through Motorex. This lasted until late 2005, when NHTSA learned not all 1990 through 1999 Skyline models would perform identically in crash testing. Motorex had submitted information for only the R33, but had asserted that the data applied to R32, R33, and R34 models.[4] NHTSA determined that only 1996-1998 R33 models have been demonstrated as capable of being modified to meet the federal motor vehicle safety standards, and that only those models are eligible for import. In March 2006, Motorex ceased all imports and Motorex principal Hiroaki "Hiro" Nanahoshi was arrested and held on $1 million bail on financial, kidnapping, and assault charges.
If you want to learn more about importing gray market cars, here are some helpful links.
Vehicle Importation and Certification Requirements
Here is a list of the cars that CAN be legally imported into the USA.
http://stnw.nhtsa.gov/cars/rules/import/elig100111.pdf
Nearly, 4 out of the 20 page document covers Mercedes Benz cars. While the importation of legal cars has slowed to almost nothing, the illegal importation of cars is still in full swing. The loop hole is that companies or individuals will import cars as "parts," and then once everything arrives stateside and clears customs, the cars are reassembled and sold without titles or proper documents. Most of which can never be driven on the roads legally in the USA.