The above formula field for week number as Apex code:

public static Integer isoWeekNumber(Date value) {
		Integer daysSince1900_01_07 = Date.newInstance(1900, 1, 7).daysBetween(value);
		Integer dayNumber = Math.mod(daysSince1900_01_07, 7) + 1;
		
		Date dateForYear = value.addDays(Math.mod(8 - dayNumber, 7) - 3);
		Integer year = dateForYear.year();
		Date year_01_01 = Date.newInstance(year, 1, 1);
		
		Integer week = (Integer)Math.floor((year_01_01.daysBetween(value)
				+ Math.mod((Math.mod(Date.newInstance(1900, 1, 7).daysBetween(year_01_01), 7) + 1) + 1, 7) - 3) / 7 + 1);
				
		return week;
	}

 and some tests:

	@isTest
	public static void knownIsoWeekNumbersWorkAsExpected() {
		Date d;
		
		d = Date.newInstance(2012, 12, 30); // Sun
		System.assertEquals(52, isoWeekNumber(d));
		
		d = Date.newInstance(2012, 12, 31); // Mon
		System.assertEquals(1, isoWeekNumber(d));
		
		d = Date.newInstance(2013, 1, 1); // Tue
		System.assertEquals(1, isoWeekNumber(d));
		
		d = Date.newInstance(2013, 1, 2); // Wed
		System.assertEquals(1, isoWeekNumber(d));
		
		d = Date.newInstance(2013, 1, 3); // Thu
		System.assertEquals(1, isoWeekNumber(d));
		
		d = Date.newInstance(2013, 1, 4); // Fri
		System.assertEquals(1, isoWeekNumber(d));
		
		d = Date.newInstance(2013, 1, 5); // Sat
		System.assertEquals(1, isoWeekNumber(d));
		
		d = Date.newInstance(2013, 1, 6); // Sun
		System.assertEquals(1, isoWeekNumber(d));
		
		d = Date.newInstance(2013, 1, 7); // Mon
		System.assertEquals(2, isoWeekNumber(d));
		
		d = Date.newInstance(2013, 1, 8); // Tue
		System.assertEquals(2, isoWeekNumber(d));
	}

 Please post here if you spot anything wrong with this for other years. Thanks.