I read they are a product of NXP. Has anyone heard of a reader that would work with the arduino?? Thanks!
Heard of MIfare some years ago (former employer), recall there was a reader with a serial interface so that should work with Arduino, but sorry no links.
Did you try the NXP website? they are pretty good in documenting things.
Google lead me to - http://www.sonmicro.com/en/index.php?option=com_content&view=article&id=88&Itemid=70 - in 14 seconds ....
Mifare cards are a type of smart card working at about 13MHz, they are used for RFID and also to store secure data on them. They are more expensive than the 125KHz RFID cards. The serial number is open access, meaning anyone can read it. It is that number that is used for RFID systems. Reading / writing data is more problematic for amateurs as there is encryption. This type of card is used by London Transport as the so called Oyster card.
Sparkfun even sells a mifare breakout board: http://www.sparkfun.com/products/10126 (http://www.sparkfun.com/products/10126). Google search is your friend.