Ok, I've just had a second member PM me and advise a similar thing has happened to their card this morning.
Check your credit card statements folks - if you find you have been affected, please PM me with your contact details should someone from the mint need to speak with you - it can help in technical investigation of any incident that may have occurred. I've advised the mint already of the issue.
I would like to point out that credit card fraud does happen every day - two reports out of 1000 visitors to Silver Stackers a day isn't damning evidence (I've had a fraudulent transaction on a card once every 2 years on average), but if a pattern emerges, it could be very useful for someone to be able to speak to you who is investigating the technical issues of any attack should it be confirmed that there was a breach.