Nintendo recently commented on the security levels of its servers amidst a devastating security breach for Sony/PlayStation Network.
At the end of April Sony reported that the network had been compromised, with around 77 million accounts stolen - including names, email addresses and passwords. Credit card details are also widely reported to have been stolen and accessed from a handful of users.
The holes in Sony's platform have left questions for both Nintendo and Microsoft, with both publishers responding to Kotaku, reassuring users that information is both digitally and physically secure. That said, no system is 100% fool-proof, so there are specialists who continue to review and monitor Nintendo's services.
Wii/DS purchases are done through prepaid cards or credit card details inputted each time players need to top up their balance, so wouldn't be stored against a user's account on Nintendo's servers.
"The security of and confidentiality of our customers’ information is extremely important to us. That’s why we have many technical, administrative and physical security measures in place to protect personal information from unauthorised access and improper use.We also review our security procedures periodically to consider appropriate technology and updated methods, and test our systems."