For U.S. passport holders, here is the exact wording for the visa requirement (taken from the Turkish Tourism website):

If you would like to get your visa upon arrival to Turkey, you will be required to have the followings:

1- Original Passport (at least valid for the next three months) (Photocopies of the passport are not accepted for processing visa)
2- A non-refundable processing fee of $100.- per person. (Exact amount of U.S. currency is required at the entry point)