How do I get a Date from a week number?

You can get Date objects representing the beginning and end of your week using the commercial method:

week = 41;

wkBegin = Date.commercial(2010, week, 1)
wkEnd = Date.commercial(2010, week, 7)

Now do your find:

  Item.find(:all, :conditions->:create_date=>wkBegin..wkEnd.end_of_day)