Agree with Pam - it depends on your use year.
If you have a February, March or April use year, then an April 2010 vacation would fall within your 2010 use year and you could borrow from 2011 if you needed more points.
If you have any other use year, then an April 2010 vacation would fall within your 2009 use year. You could borrow from 2010, but not 2011.