She may very well have a UTI. She may be in pain. She should be taken to the vet tomorrow.
If that turns out not to be the problem, them there are a whole lot of other things to look at, such as: number of litter boxes, where they are, type of litter, relationship with other cat, cleanliness of litter pan, type of soap used to clean pans, type of litter pan, stress of household, recent changes, etc.
But most often a UTI is the cause, so start there.
Robin
Edit: You replied while I was writing. I would go back to the vet, or to another if you have doubts about the first one. This behavior is for a reason. Something is very wrong.
On the short term, I'd confine her to a small room where she can't damage anything. Provide a low sided large litter pan - 2 if possible - with non clumping clay litter (no scent). Scoop very often, and wash pans and change litter every couple of days. Give her a comfy bed, like an old towel, and spend time with her there. She needs retraining, as well as medical care. The vet may need to do more extensive tests.
Edited by bastetservant - 2/15/12 at 5:52pm