According to information translated by the genius behind GoNintendo.com the ranch will be owned by Yukari, a friend of Bebe from Pokemon Diamond/Pearl.
You'll be able to upload Pokemon from your DS into the Wii, storing up to 1,000 in the ranch. You can then also take photos of them and send them to the Wii Message Board and presumably to other peoples Wii's.
Apparently there will also be a number of activities you can use to interact with your Pokemon but little is known about these at the moment.
While this download may not appeal to everyone it sounds like the perfect accessory to Pokemon Diamond/Pearl.
The release date and price remain unknown at the moment.