Is the nissan skyline legal in the united states?

Yes and no. Some are legal and some are not. Basically the R32 and maybe R33 (not sure) are not legal and the 99-01 R34 is legal. The reason for this is that the US Department of Transportation does not have crash data for these cars. Although due to all the popularity of the R34, the DOT has enough crash test data that they have deemed the car safe enough for road use in the US. But im sure you will have the DOT breathing down your neck if you dont have it converted to left hand drive...but...if you have the cash laying around, and you import one through MotoRex they will convert it over for you, but its not cheap.