I don't really care if someone is talking on a cellphone in the bathroom, unless:
(1) They get nasty about other people's "bathroom sounds," e.g. flushing, water running, whatever.
(2) They accuse others of eavesdropping on their conversation. I had this happen when I was in line at the bank once; the woman in front of me was talking on her cellphone, and so I was trying to daydream so I wouldn't listen to her conversation. At one point she turned to me and snarled (yes, snarled), "Do you mind? This is a private conversation!" At that point she was called to the teller, who told her she had to hang up or he couldn't do her transaction. She got all upset about that, but the bank manager backed the teller up. She ended up stalking out, still talking on her phone, leaving a stream of curse words in her wake.
(3) They block access to the toilets or sinks. They don't have to be in a stall like it's a phone booth, or standing in front of the sink; they can just as easily go out into the hallway (if there is one) or wherever to finish their conversation.
I've heard conversations consisting of *ring* "Hello? Okay, I'll be home in about an hour. Love you. Bye." Click. Those don't bother me at all, and in fact I think that phone calls conducted in public should be like this, and the long, involved calls should be done in private.