If you are getting December 2008 points, they will expire on November 30, 2009. You will have up until July 31, 2009 to bank any amount of them if you wish. (If banked, they then would expire on November 30, 2010.)
The only reason they can't be used until May 2009 is that's because the unit you bought into will not be open until then. That does NOT mean that you can't make a reservation until then. You will be able to make any reservation you want using the regular reservation windows, as long as the stay does not begin until whatever date in May that your unit is available for use.