http://jsfiddle.net/F6qhb/

random.js

//@param items {Array} ランダムに抽出する対象の配列
//@param pickoutNum {Number} 抽出する個数
//@see http://d.hatena.ne.jp/miya2000/20080607/p0
function random(items, pickoutNum) {
    var tmp = {};
    var result = [];
    var itemNum = items.length;
    pickoutNum = (pickoutNum < itemNum) ? pickoutNum : itemNum;
    while (pickoutNum-- > 0) {
        var i = Math.random() * itemNum | 0;
        result[pickoutNum] = tmp[i] || items[i];
        --itemNum;
        tmp[i] = tmp[itemNum] || items[itemNum];
    }
    return result;
}

console.log(random([0,1,2,3,4,5],2)); // 0~5
console.log(random([0],2)); // 0
console.log(random([0,5],2)); // 0,5
console.log(random([0,1,2,3],2)); // 0~3