Yes, this happens to me all the time. I usually either hit the mute or hang up with my face. After reading the forum and experimenting I understand why it happens on the Evo 4g.
The proximity sensors are located under the htc logo and in the center between the screen and the earpiece. However, there are no sensors next to the camera lens which is in the top right corner. So, when I hold the phone in my left hand and tilt the phone away from my ear with the back of the phone forward, it lifts the sensors by the htc and center away from my ear, and because there are no sensors by the camera (which would be next to my ear) the screen activates. Then, any touch from my face or ear mutes it, activates the speaker, or hangs it up.
A quasi solution is to hit the home button at the beginning of each call so that the phone app does not appear when the screen is activated. However, lifting the right side of the phone away from your ear will still activate the screen which uses up battery power, and your face might then hit the phone app which brings it back up and the problem may still occur.
Or, you could develop the habit of holding the left side of the phone (the htc logo) next to your earlobe. This seems to keep the sensor activated and the screen off.
Unless HTC adds sensors to the right top of the phone, a new phone will not fix the problem.