Thank you!

I found some errors in the code:
Code:
var I:Number = 365;
if (isLeapYear)
{ 
   var I:Number = 366;
}
It should be:
Code:
var I:Number = 365;
if (isLeapYear)
{ 
  I = 366;
}
I have added some declarations of some variables to avoid varnings.

My version:

Code:
/**
 *	ISO-8601 Week Number - V 1.1
 *	Processes the week number of a date,
 *	just like date("W") would do in PHP.
 *
 *	Procedure by Rick McCarty, 1999
 *	Adapted to CVI by R.Bozzolo, 2006
 *	Adapted to AS2/AS3 by A.Colonna, 2008
 *
 */
function calculateISO8601WeekNumber(d:Date) {
// 1) Convert date to Y M D
	var Y:Number = d.getFullYear();
	var M:Number = d.getMonth();
	var D:Number = d.getDate();
	
	// 2) Find out if Y is a leap year
	var isLeapYear:Boolean = (Y % 4 == 0  &&  Y % 100 != 0) || Y % 400 == 0;
	if (isLeapYear)
	{
		trace('Is a leap year');
	}
	
	// 3) Find out if Y-1 is a leap year
	var lastYear:Number = Y - 1;
	var lastYearIsLeap:Boolean = (lastYear % 4 == 0  &&  lastYear % 100 != 0) || lastYear % 400 == 0;
	if(lastYearIsLeap)
	{
		trace('Is year after a leap year');
	}
	
	// 4) Find the Day of Year Number for Y M D
	var month:Array = [0,31,59,90,120,151,181,212,243,273,304,334];
	var DayOfYearNumber:Number = D + month[M];
	if(isLeapYear && M > 1)
		DayOfYearNumber++;
	trace('Day of the year: ' + DayOfYearNumber);
	
	// 5) Find the weekday for Jan 1 (monday = 1, sunday = 7)
	var YY:Number = (Y-1) % 100; // ...
	var C:Number = (Y-1) - YY; // get century
	var G:Number = YY + YY/4; // ...
   	var Jan1Weekday:Number = Math.floor(1 + (((((C / 100) % 4) * 5) + G) % 7));
	trace('Day of the week for Jan 1 (1 = monday, 7 = sunday): ' + Jan1Weekday);
	
	// 6) Find the weekday for Y M D
	var H:Number = DayOfYearNumber + (Jan1Weekday - 1);
   	var Weekday:Number = Math.floor(1 + ((H -1) % 7));
	trace('Day of the week for date (1 = monday, 7 = sunday): ' + Weekday);
	
	var YearNumber:Number = Y;
	var WeekNumber:Number;
	// 7) Find if Y M D falls in YearNumber Y-1, WeekNumber 52 or 53
	if (DayOfYearNumber <= (8-Jan1Weekday) && Jan1Weekday > 4)
	{
		trace('Date is within the last week of the previous year.');
		YearNumber = Y - 1;
		if (Jan1Weekday == 5 || (Jan1Weekday == 6 && isLeapYear))
		{
			WeekNumber = 53;
		} else
		{ 
			WeekNumber = 52;
		}
	}
	
	// 8) Find if Y M D falls in YearNumber Y+1, WeekNumber 1
	if (YearNumber == Y)
	{
		var I:Number = 365;
		if (isLeapYear)
		{ 
			I = 366;
		}
		if (I - DayOfYearNumber < 4 - Weekday)
		{
			trace('Date is within the first week of the next year.');
			YearNumber = Y + 1;
			WeekNumber = 1;
		}
	}
	
	// 9) Find if Y M D falls in YearNumber Y, WeekNumber 1 through 53
	if (YearNumber == Y)
	{
		trace('Date is within it\'s current year.');
		var J:Number = DayOfYearNumber + (7 - Weekday) + (Jan1Weekday -1);
		WeekNumber = J / 7;
		if (Jan1Weekday > 4)
		{
			WeekNumber--;
		}
	}
	
	return WeekNumber;
};