[Stripes]

Go to the WDW site, sign-in and hover over My Disney Vacation in the upper right corner. You should get a drop-down menu with several options. At the bottom is My Reservations > Dining Reservations. Currently, all you can do is view and/or cancel.

UPDATE: Reservations can now be MADE ON-LINE for certain restaurants!!!
To make reservations on-line:
- go to the WDW Restaurant Page Use the links on the left to narrow your choices by location.

I'm editing to add a list of restaurants where ADRs can now be made on-line.
Lists are by park / resort, then alphabetical.

Parks

Magic Kingdom:

Cinderella's Royal Table
Crystal Palace
Diamond Horseshoe (?? - thought that was seasonal)
Liberty Tree Tavern
The Plaza
Tony's
Tomorrowland Terrace

Epcot

Akershus
Biergarten
Bistro de Paris
Coral Reef
Garden Grill
Le Cellier
Les Chefs
Marrakesh
Nine Dragons
Rose & Crown
San Angel Inn
Teppan Edo
Tokyo Dining
Tutto Italia

Animal Kingdom

Rainforest Cafe
Tusker House
Yak & Yeti

Disney Hollywood Studios

50's Prime Time Café
Hollywood Brown Derby
Mama Melrose's
Sci-Fi Dine-In Theater

Resorts

Animal Kingdom Lodge
Jambo House

Boma
Jiko

Kidani Village

Sanaa

Beach Club Resort

Cape May Café

Boardwalk

Flying Fish Café
Spoodles - currently on the list but scheduled to close

Contemporary

Chef Mickey's
California Grill
The Wave

Coronado Springs

Maya Grill

Dolphin

Shula's Steak House

Fort Wilderness

Hoop-Dee-Doo Musical Revue
Mickey's Backyard BBQ
Trail's End

Grand Floridian

Citricos
The Chef's Domain at Citricos
Narcoossee's

Old Key West Resort

Olivia's Café

Polynesian

'Ohana
Kona Café
Spirit of Aloha

Wilderness Lodge

Artist Point
Whispering Canyon Café

Yacht Club

Captain's Grille
Yachtsman Steakhouse

Downtown Disney

There are currently no DTD restaurants available to book on-line.