This has been a controversial question since the beginning. During BB1, they claimed that the toilet-cam was there "for the HM's safety", and they would only broadcast footage that was relevant to storylines: e.g. when discussions take place in the toilets (as BB6's Lesley discovered to her embarrassment!) This still implies that the toilet-cam is on 24/7, though...
Interestingly, the only significant change made to the house for CBB1 was that the toilet-cam was
removed. Presumably BB regards the safety of celebrities to be less important than that of the rest of us...