Almost all our cameras are installed in very rough areas. We have never had anybody try a sledge or baseball bat. Most people vandalizing the cameras actually try to steal them and sell them. One guy actually spent a couple days inspecting and finding the correct tools to take them down. He successfully stole three from the hallway of his apartment complex. We have video of him exiting his apartment, setting up his ladder, and taking down the camera, all while staring straight into the lens (people never cease to amaze). There is not a lot you can do if someone is going to spend that kind of time. However, most guys just go at them with a crow bar. In that case, I'm usually more concerned with the backing material than the camera. Nothing will stop the determined.
It does seem that IP camera manufacturers tend to use thinner domes than I'm used to from analog vendors like Speco. My guess is that thinner domes provide better optical characteristics.
While I'm sure there are some crazy robust enclosures out there, a can of spray paint is a cheap way to render any cam useless.