You are currently in your Dec 2009 UY and an April 2010 reservation would fall in your Dec 09 UY. If you don't have any points left in your Dec 2009 UY, then yes you would have to borrow points from your Dec 2010 UY to make the reservation.
as stated an april 2010 vacation is in the dec 2009 use year. for an april 2010 vacation, you can use:
*banked dec 2008 pts
*current dec 2009 pts
*borrowed dec 2010 pts
you can call 11 months in advance (in may 2009) to book your home resort with any combination of the above pts, and 7 months in advance to book any other DVC resort.