Getting the ISO weeknumber in SQL Server 2005 (and earlier)

I just discovered that SQL Server often returns the wrong week number when using DatePart(wk, @date).
For instance, SELECT DatePart(wk, '2010-12-17') returns 47 while it should be 46.
This is because SQL Server starts counting weeks from 1 Jan, so 1 Jan is always in week 1 of the year.
The ISO standard states that week 1 is the first week with 4 days in it.
The following code can be used (with @date being the datetime) to return the ISO week

CREATE FUNCTION GetISOWeek(@date DateTime)
RETURNS INTEGER
AS
BEGIN
  declare @ISOweek INTEGER;
  select @ISOweek = datepart(wk ,@date) + 1 - 
    datepart(wk, 'Jan 4,' + CAST(datepart(yy, @date) as CHAR(4)));
  if (@ISOweek = 0)
    select @ISOweek = datepart(wk, 'Dec ' + CAST(24 + 
      datepart(day, @date) AS CHAR(2)) + ',' + 
      CAST(datepart(yy, @date) - 1 as CHAR(4))) + 1
  RETURN @ISOweek
END
GO
(From windowsitpro.com)

Since SQL2008, DatePart supports the ISO Week, by calling:
DatePart(isowk, @date) or  DatePart(isoww, @date)