According to an article published on Siliconera, the new Nintendo point cards that will be available for use on the Wii Shop channel or the DSi channel (when it’s available) must be used in one of the two systems at a time.

Basically, it comes down to choosing with platform to use the card. There will not be an account that allows players to share the points between Wii and DSi and vice-versa, so each card must be applied on the platform the user wants to purchase things from. In that sense, it’s also easy to assume that credit-card point purchases won’t be transferable either.