I've been using this calendar to have week number in google calendar : http://sites.google.com/site/gcalweeknumbers/googlecalendarweeknumbers
But since a few weeks it fails to load. I try to re-add it without success from the webpage.
I managed to make it work again by tweaking the url :
replace GOOGLEAPPSDOMAIN.COM by your google apps domain name, paste the link in a browser that have a google apps session open, it will ask for comfirmation before adding this calendar to your calendar, click yes and you have weeks number back in your caledar!