As others have said, busses are wheelchair accessible.
Not only that - almost all transport at Walt Disney World is accessible including the monorail and boats. There are a few exceptions, like the smaller boats that go between Magic Kingdom and the hotels surrounding the lake (you have to step down into the boat). A few of the larger boats have problems loading/unloading wheelchairs if water levels in the lake are not right.
Even Magic Express from the airport and Lynx (Orlando public transport system) are wheelchair accessible. Ther are even wheelchair accessible taxis operating in and around Disney property and Orlando.
Wheelchair access is one of the reasons I almost always vacation at WDW.
Andrew