Let me clarify this: In movie mode (or live view), the sensor is gathering light all the time, and due to this the heat starts to build up.
This is a problem in certain situations, due to the heat produces more noise and sometimes the heat is so intense that the camera turn off itself for security reasons.
I've heard a rumor that if you enter the menu while in movie mode, the sensor gets "deactivated", reducing the heat on it. This is very convenient due to switching from live view to the normal mode back and forth has some impact in the shutter.
Is this true? How do you prevent heat without leaving the live view?