Trips are based on the UY that they take place and each night is treated on its own.
With a Dec UY, and a trip started November 30th, you will cross your UY so what you can use for November 30th will be different than the rest of the trip.
So, if you want to stay November 30th, 2012, that will take place in your Dec 2011 UY--you can used banked 2010 UY points, 2011 UY points, and borrowed 2012 UY points to pay for that one night. Any bankable points you have in your account now would be 2011 UY points and would not have to be banked for this ONE night.
The rest of your trip, starting Dec 1, 2012, will take place in your 2012 UY. You will be able to use banked 2011 UY points, 2012 UY points, and borrow 2013 UY points for these nights.
So, if you have 2011 UY points that you want to use for these nights, then yes, you will have to bank them, before you make the reservation.
Good luck!