I want to do a version of that which begins with the credit card types desaturated and then colors the one that matches the number entered (for which there are Free js libs)
Also, the credit card number series should be tightened up when automatically selecting card type. Wikipedia actually has a nice list of the series (http://en.wikipedia.org/wiki/Credit_card_number).
A few of the larger merchants can benefit from choosing a different transaction acquirer based on card type (e.g. lower fees for Amex) but they should not trust the card type selected by the customer.
This is similar to the way PayPal does it, which I've always thought is a great, subtle way of confirming the card type.