In Schiphol Plaza there is a mobile phone rental store run by KPN Telecom.
http://www.rentcenter.nl/Homepage_eng/homepage_eng.html
People with a US GSM account on T-mobile, ATTWS or Cingular WITH roaming activated on their accounts can either use their own phone (if they have a tri-band GSM phone) or they can rent a phone at the rentcenter and put their own sim in it.
People without a phone are probably cheaper just buy a prepaid phone in the Netherlands as the rentals are €43 a week.
Orange has the cheapest prepaid phone packs as you can see here:
http://eshop.orange.nl/itemList.asp?categoryID=3