Checker Cab (a Mears company) is the "official" cab of WDW... you'll find them everywhere.
I beleive the rates are $2.00 for the first 2/7th of a mile, and 25 cents per 1/7th mile thereafter. Also 25 cents for every 30 seconds wait time. Hrmm... those aren't the exact rates, but they're somewhere around that.
407-699-9999 is the phone number.
One thing to watch out for... make sure your driver is a good driver... obeys the speed limits, stops for red lights, and stop signs... etc... There are a lot of cab drivers out there that are horrible... I can't tell you how many times I've been in close encounters with them. So, please, be careful.