Not all cat litter is unsuitable, I think it is the clumping varieties you need to avoid
Bio-catolet is the best litter I have found, and I have used it for the rabbits and the cats. It's expensive though, I just use newspaper and hay now.
ETA: If using any kind of litter, you need to make sure that your bunnies aren't eating it, some bunnies do and some don't. If they are eating their litter, then even the suitable for bunnies kinds could cause a problem