Follow the link in my signature to the disABILITIES FAQs thread on the disABILITIES Board.
Post #1 of that thread is an index to the other posts in that thread. One of the posts is about bus transportation and post #3 has a lot of information about in the parks.
You will be able to bring the wheelchair into all attractions up to the boarding area and all shows have accessible seating areas.
Most of the regular lines are totally wheelchair accessible, so there are few times where you will not enter thru the same entrance as everyone else. These are called Mainstream Lines; AK and the Studio were built with Mainstream Lines. MK and Epcot have some lines that are not Mainstream, but Mainstream Lines were added as attractions were added or renovated, if at all possible.
For some attractions, the line is accessible, but the boarding area is not. For those, you will see a wheelchair bypass (marked with a chain or gate with a wheelchair symbol) either by very close to the regular boarding area or close to whatever makes the line inaccessible.
WHen you enter each park, they have park maps in kind of a kiosk. Look there for a park map for Guests with Disabilities. That map has information about boarding rides that is not on the 'regular' maps that will tell you how each attraction workd.If you don't see the maps, you can pick one up in Guest Relations.
My DW had surgery on her foot and can only walk short distances, so we rented a wheelchair. At the bus stops we will fold it up like a stroller and should be OK. What is the policy in the park for rides and shows? Never had this situation. 
