A number of sources, including Touring Plans.com, are reporting that Disney has quietly begun allowing advanced dining reservations ("priority seating") to be made online.
You can pick the restaurant, date (or "range" of dates, great if you know you want to eat at a certain place at a certain time but are flexible in doing it any day of your trip), and number of people in your party. If your selected time is not available they'll give you the closest time on either end and also give you "suggestions" of other places available nearby and close to your time. This is a far more realistic suggestion system than the one for resorts where a search for a room at Pop Century will yeild a suggestion of a suite at the Polynesian.
It appears that you can't make an ADR less than 3 hours in the future. It's about 4:30 right now and I got an error message for 6:00 and 7:00 searches for tonight but got returns for 8:00. Not sure if that's an anamoly or if its the policy.
The reservation page also includes the note: Online dining reservations at Cinderella's Royal Table and Chef Mickey's are currently available for dates on or after June 20, 2009. For reservations dates prior to June 20, please call (407) WDW-DINE.
You can check out the system online at Disney World.com. Interestingly the restaurants they are promoting on the main page are The Wave, Citricos, Artist Point, Captain's Grill, Rose & Crown, Biergarten, and Olivia's. Don't know if that's a random sample, a list of places where plentiful ADR's are available, or actually what Disney thinks are the best representations of its restaurants.
I just did a quick check of a couple of spots to see how it works (and just for kicks to see what's booked at this point and what isn't.) I searched for a month out, July 16. San Angel Inn for dinner at 6:00pm was booked but was available at either 5:45 or 6:15. Le Cellier was completely booked for the night. 'Ohana (which a search of simply "Ohana" did not return) is booked for breakfast at 9:00am but available at 8:50 or 10:10. Liberty Tree Tavern was booked for lunch at noon but available at 11:55am or 12:30pm. They were also booked for dinner at 6:00 but available at 5:25 or 6:35.
Anyway, that really served little purpose but it was fun if you're like me and interested in knowing which restaurants fill up when. If you're curious, if you want to eat dinner at Epcot tonight there's an 8:15 at Bistro de Paris, an 8:45 at Nine Dragons, and Tokyo Dining at 8:55.