Note: On January 8, 2016 American Express sent out a notice to a large number of Bluebird & Serve cardholders informing them that loading capabilities on their accounts had been terminated. For more information, see: Amex kills Bluebird and Serve for manufactured spend
American Express has announced via the help section on their website that they will no longer accept Visa, Mastercard or Discover cards to load the Serve card after April 15, 2015.
They will still allow American Express cards to be used, however American Express cards do not earn points for those types of loads.
According to the site:
Note: Starting April 16, 2015 you will only be able to use an American Express® Card if you want to load your account with a credit card. Discover, MasterCard and Visa credit cards will no longer be supported after April 16.
Update: According to some readers, third party American Express cards such as the Fidelity Amex do earn rewards on credit card loads. Based on the language from the American Express website, these cards should continue to work.