VISA is accepted, just not over the phone -- although I thought this had changed recently... The last time I paid with a MC, I had to fax the payment in or return the payment coupon that came with your ressie with your VISA/MC info on it. I think they only took AMEX over the phone to encourage people to use that card... but like I said, I thought this had changed recently.
VISA is certainly accepted throughout WDW as a form of payment at all hotels, restaurants, and shops.