I've been told up to a year in advance, but recent reports indicated that they weren't taking reservations past 12/31/04. I've also heard that WDW is upgrading some computer systems, so that may tie-in to the inability to book too far in advance.
Call and ask. I've had three or four occasions to call the tour line in the last 6-8 months, and found that they answer callers very quickly. The number is 407-wdw-tour.