Well, Nintendo will release a special Flash Card that you'll just insert into the DS Slot, fire it up, and it'll launch the service where you can buy GB games, initially... and then later down the road, Gameboy Color games. I'm assuming, way later, you'll be able to get GameGear games, etc...
Finally, Nintendo does what I've been predicting for a year. Took them long enough though. Yay...I can finally play Link's Awakening DX and Metroid II.