The new Nintendo Points cards will force you to pick a platform when it's redeemed. Siliconera reports that the House of Mario currency will not go into one main Nintendo piggy bank for the user, but will need to be redeemed for either DSWare or WiiWare titles.
Another way of putting it is that consumers won't be able to transfer points between the Wii and DSi after activating the cards. It's a shame really, we certainly expected a more 21st-century solution (i.e. an all-purpose Nintendo account) from the company which brought us the brilliant 16-digit friend code system.