There's only one contract between Sprint Nextel and Clearwire. If there is Clearwire service in an area, and you've got a WiMax phone, you can use it.
As Leslie Ann says, Clearwire is insanely optimistic about coverage. I'd even go so far as to say that Clearwire misrepresents on their coverage map.
Sprint and VM have their own WiMax coverage maps and they are much more realistic about where you can get a signal, even breaking it down into indoor vs. outdoor. I'd trust the Sprint/VM maps over the Clear maps any day.