According to Neowin, many people that took advantage of Microsoft's Vista Family Discount were issued invalid keys. The problem has been escalated to a program manger but, in the mean time, those people are just stuck:

Microsoft says, "This is a known issue and has been escalated to the program manager." When asked what could be done I was told, "I can take your name and number and call you back when there is a solution. If that is not acceptable, I can cancel your order and issue you a refund." I was also told, "There is no expected time period for a fix at this time."

If Microsoft does not have this issue fixed very soon, they are going to have a lot of unhappy customers. In the past Microsoft has just issued new keys when this happened. Why is Microsoft being so tight gripped about handing out new keys? If I purchased a valid key, what is it for? How long is this going to take to fix? So many questions, so little answers...