From wdwinfo.com

Starting in January 2009, guests staying at select Disney resorts will be able to check-in online up to 10 days prior to arrival:

The online check-in will include providing the credit card to be used for room charges, your arrival and departure times, your address, on-site (cellphone) number, names of all guests and your room requests (these are subject to availability and will not be guaranteed.) Part of the process will include reviewing and agreeing to the registration terms.

Your time of arrival will not impact your ability to use the online check-in. If you arrive early, there is no guarantee that your room will be ready. While efforts will be made to have the room available, standard check-in times remain in effect.

Upon arrival there will be signage in the resort lobby indicating where the online check-in area is located

A valid photo ID, which includes a drivers license, govt. issued passport, govt. issued photo ID or active military ID, will need to be shown
You'll be given a welcome folder containing your resort/Disney World information and your room keys

This online service is for check-ins only. If you need to modify/change your reservation, you must use the same method you used when you booked your reservation.

If you booked either through the Disney Reservations Center or with Disney Cruise Line, you will get online check-in information either via an email, online or with your confirmation documents.

If you booked via the Disney Vacation Member Services, you will find online check-in information on the Member website, the Disney Files magazine and through member services. If you booked through a Travel Agent, you will be able to check-in at mydisneyreservation.com once the program is active.