If you're 25 or over, I'd highly reccomend a car.
What my fiancée and I did when we went in Sept of 2002, we picked a car up at the airport. We went to Universal Studios, and that night checked out the Boardwalk, ate dinner off-property, and picked up some groceries. We then dropped the car off the next day at the Doubletree in Downtown Disney, and we reserved a one-way trip with Mears to get us back to the airport. We caught one of the busses from the Doubletree to the parks. There's a few different rental agencies scattered about the property.
Budget is at the Doubletree, Dollar is at the Holiday Inn, and I think there's something at the Hilton....
Avis maybe? I think
Alamo and National are at the Swalphin, and National is available at the Car Care Center outside of the Magic Kingdom parking lot.
I think it cost us about $70. But I really didn't scour for any deals, so I know I could have done better. But it was SO worth that $70 for the convenience we enjoyed.
If you're 24 or under, I'd skip it. Those underage driver fees are killer.