As I understand it, "Guarantee" statements are generally a legal matter - it depends on whether the money is "on hand" and set aside upon your entering the program versus pulled from ongoing operating costs.
Realistically, I don't think it makes a big difference. If a program tells you that at no point in the last 20 years has anyone ever not been funded, I wouldn't freak out if they won't formally guarantee it. My program doesn't guarantee...I'm not worried, even with the economy being what it is. Its generally expected that you will have a tuition waiver with a stipend in the clinical world, and I'm always amazed how some programs manage to draw enough applicants to survive (though apparently PsyD programs somehow manage). The schools that don't offer funding are definitely a minority.