I think the DS will be a great system for sports.
Imagine being able to design your own plays with the stylus touch screen...even to the point of designing the plays during the huddle!
If that is too ambitious you can atleast pick plays quickly and easily, and then have quick choices for audibles and such. And the same customization can be placed into all sports games. Formation changes on the fly, quickly call individual players in baseball to cover the field more conservatively or aggressively and so on and so on.
Nintendo doesn't need to make these games themselves. Nintendo just needs to sell the concepts to EA. If EA sees potential in the system the games will come.
What I really hope is that Nintendo spends some money like they are in Japan and creates DS hubs around America for meeting places to play games.
Ideal places would be:
1)McDonalds (They already have the N64s in store.)
2)Find a large coffee house chain, or bookstore chain that wouldn't mind having a seperated off section for this very idea.
3)College Dorms. If Nintendo spent some money and created a HUD and HUD boosters within college Dorms then it would become an instant success for multiplayer gaming.