Actually, I have been told by CMs who have been involved with room assignments at the resort that they pay more attention to the requests that are called in than the ones that are faxed.
Your mileage may vary- some guests have reported being successful with faxes, particularly when they are part of a subscription service, but our recommendation is to call in your request through Disney's Central Reservation Office.
These are the numbers I have listed in the FAQ/Other Useful Info Section- the second number ending in 1936 is the one listed on the WDW phone directory:
Walt Disney World (number on wdw website): (407) 939-5277
7 am to 11 pm
Walt Disney World Resort Reservations: (407) 939-1936
0800 16 90 730 (UK)