This page provides an iCal format description of the University of York week numbers, according to the university term dates calendar. It is compatible with Google Calendar, and can simply be added to your view by subscribing to the following URL:
For week numbers for the whole year, see http://sites.google.com/site/gcalweeknumbers/. This will show the week numbers as blue circles (as shown below) and was the basis for this calendar.
Google also have their own week numbers under Add -> Browse Interesting Calendars -> More.
The source code used to generate the calendar is available on Github.