If your card is stolen, you should report it immediately and it will be deactivated. Where this idea would be really helpful is against identity thieves, who could charge your account without even getting your card.

i think the e-mail idea is good, unless you use your card too much, then your inbox will be full, because i think that most people may go a day, even two without even looking at their card, and without knowing if it has been lost or stolen for this time, but they certainly check their email. it could also be a great way for parents to keep track of the so common "emergency card", most teens will use this card for little things, then by the end of the month they can say they forgot what it was, if the cardmember, ie. the parent recieves instant notification, they can immediately ask what it was for and prevent abuse.

In Europe some companies send you a text message to your cell immediately after the transaction is made. I think email or IM are equally effective, and I think it's a good idea to detect undesired charges right away.

instead of email why not sms think about it/

you will be able also to define in which case you want to notified