The best setup I have seen was on the Woodlands custom homes tour about 6-7 years ago. They had a TV mounted behind a two-way mirror so when you turned the tv on, it came on in the middle of the two sinks in a full length mirror. Of course, you have to have external speakers.
We did this in our master bath, but we built a corner cabinet and put a 14" TV behind a door, so when the door is shut your tv is hidden. We can see it from the sink, tub, and shower.
As for your original question, I would think mounting over a shower would be a bigger issue with all the steam. I probably would not mount it directly over a tub, but if you were a couple of feet away, and have a exhaust fan in the bathroom, I would not think it would be a problem.
aggiez