Strangely enough, Mathf.PingPong is really good for 'ping pong' effects. ;)

var rotateSpeed = 50.0;

function Update () {
    transform.rotation = Quaternion.Euler(0.0, Mathf.PingPong(Time.time * rotateSpeed, 90.0), 0.0);
}

You could always do:

function Update(){
if(transform.eulerAngles,y = 0) {
transform.eulerAngles.y = 90;

} if(transform.eulerAngles.y = 90){
transform.eulerAngles.y = 0;
}
}

You could also use quaternion.fromtorotation to get this effect. Finally instead of transform.eulerAngles.y = 90 or 0, you could use mathf. lerp from 90 to 0 or 0 to 90.