If August use year:
1. Points issued Aug 08 must be used for a trip that occurrs between 8/1/08 and 7/31/09 unless you bank. You have until March 31, 2009 to bank any or all of those points in the Aug 08 use year and you can bank without making reservation at the same time. If you do not bank any remaining points in that use year by 3/31/09, you cannot do so at all after 3/31/09, e.g., you cannot call in April 09 and use them for a trip in February 2010 because you will be prohibited from banking them. You must commit the positive act of banking to get points banked; MS does not bank points automatically just because you are reserving a trip in a future use year.
2. Assume you have Aug 08 points and call today to make reservation for December 09. The only way you can use those Aug 08 points for that trip is to bank them first (on same call as making reservation or earlier) into the Aug 09 use year. Date you call to reserve does not determine points used for a trip. Dates you will be at WDW determines that. For that Dec 09 trip if you were calling today, you could use Aug 08 points if and only if you bank them. You can use Aug 09 points which will be "current" use year points for a Dec 09 trip, and you could borrow, if needed, points from your Aug 2010 year.