It's an unfortunate reality of the cyber-world that hack attacks are a fairly regular occurrence, with Ubisoft recently falling victim. The most high-profile example — in the gaming world at least — was the infamous attack on the PlayStation Network, taking the service down for a month and, with it, online gaming on the PS3.
Nintendo is vulnerable to attacks too, of course, and Club Nintendo in Japan seems to have been the target of a hack in recent days. A notification on the region's customer support page advises Club Nintendo members to change their passwords. A translation of the page states that investigations started on 2nd July, and suggests that between 9th June and 4th July 23,926 accounts were illegally accessed; this was reportedly from 15,457,485 attempts.
The message states that details such as name, address, phone number and email addresses may have been compromised; it's made clear that no credit card details were stored or at any risk. This does appear to be a Japan-only issue, with Nintendo telling CVG that "this only affects the Japanese Club Nintendo and is not an international issue".
Naturally users in Japan should change the password on their account, and this is certainly an unwelcome development for Nintendo. We'll keep you up to date if there are any further developments.