function evenRound(num, decimalPlaces) {

    var d = decimalPlaces || 0;

    var m = Math.pow(10, d);

    var n = +(d ? num * m : num).toFixed(8); // Avoid rounding errors

    var i = Math.floor(n), f = n - i;

    var e = 1e-8; // Allow for rounding errors in f

    var r = (f > 0.5 - e && f < 0.5 + e) ?

                ((i % 2 == 0) ? i : i + 1) : Math.round(n);

    return d ? r / m : r;

}



console.log( evenRound(1.5) ); // 2

console.log( evenRound(2.5) ); // 2

console.log( evenRound(1.535, 2) ); // 1.54

console.log( evenRound(1.525, 2) ); // 1.52

Live demo: http://jsfiddle.net/NbvBp/

For what looks like a more rigorous treatment of this (I've never used it), you could try this BigNumber implementation.