Beginning in late July, early August of this year it's rumored that you will not be able to make dining reservations past January 15, 2009 (or 25th - depending on which rumor you're reading).
This magical date in January will be the universal cut off for reservations for a period of time.
Beginning in October of this year, when it's 90 days from January 15 (or 25) you will be able to make reservations for dates past the magical date in January.
It is rumored that they will keep this 90 day window while they are upgrading their ADR system and beginning in late 2009 the window will open back up to 180 days.