They're close, but not always standard. If you had an Acer DVD drive, and you picked up an Acer CD drive, you'd probably have better luck then with a different brand. I've snapped them off, and not have actually broken anything on them and been able to replace them, but I know it was difficult.
When (if) I get a DVD drive, it'll be slot load for this very reason. Computer case on the floor is just too dangerous 😛