I got my PhD without one dime of debt, and I think most of my classmates did as well. There is a lot of funding for grad students if you are flexible about what you want to work on. And if you absolutely MUST write your thesis on optimizing brainfuck compilers then you can still cover your tuition by working as a TA.

But this may just be the situation at my alma mater, a land grant university that attracts lots of research dollars but doesn't quite have the prestige to attract lots of talented students. I wonder if students at "name brand" schools don't end up with more debt because competition for funding is more intense.