As a current Cambridge student, you can apply for the scholarship if you apply for admission to a new program (ie, you are doing an MPhil and applying to start a PhD). When I applied for my program this year, all you had to do to apply for the Scholarship was check a box and submit an extra 500 word essay by the funding deadline (October 15 in the US), and they consider you for the scholarship. See the FAQ: http://www.gatescambridge.org/apply/faqs.asp
I have not heard back yet either.