As others have mentioned, it's not in the 3rd edition specification. It is in the 5th edition specification however, and I quote:
ECMAScript defines a string interchange format for date-times based upon a simplification of the ISO 8601 Extended Format. The format is as follows: YYYY-MM-DDTHH:mm:ss.sssZ
So it should be trickling into browsers soon (Microsoft have developed a new JS engine for IE9, it makes sense that it would use ECMA 5th). If you want to implement a solution in the meantime, you might want to optimize so that your script can take advantage of the native if available:
(function ()
{
if (isNaN(Date.parse("2010-02-23T23:04:48Z")))
{
var oldParse = Date.prototype.parse;
Date.prototype.parse = function (strTime)
{
// regex test strTime for ISO 8601, use oldParse if it isn't
// Use custom parser if it is.
}
}
})();