Is it harder to get into PhD programs for some disciplines compared to others? Anecdotally it seems much harder to get into a good math program than a computer science program. How true is that? Are there any statistics available on the selectivity of different disciplines?
Statistics might be hard to come by, but you can always checks the requirements of the Phds on the webiste of universities. 

In my alma mater (Canada), two were particularly easy to enter: psychology and semiotics. You could start them directly after a simple bachelor degree, you did not have to go trough a Master beforehand and, the teachers would try to entice you to do one. Anybody could do one even with relatively bad grades.