I think there are. I'm pretty sure there's one at the Magic Kingdom to the right of Guest Services outside the park; at either MGM (I think), it's to the left while you're waiting to get in... somebody with park maps handy will be able to give you definite answers.
At MK, once you're through the main gates but waiting for the rope drop, on mornings when they don't open Main Street before official opening, there aren't bathrooms. Learned that the uncomfortable way.
This is a site from another Diser---hope it's okay with her that I'm reposting. She did a GREAT job taking pictures and describing the location of all of the women's bathrooms. http://www.cassworld.ca/photosbathrooms.htm
How abount baby changing facilities in each bathroom? I know they have a special baby spot, but waht about just the one changer thing on the walls? Do they have them in each bathroom?