Electrical components have a habit of converting electrical energy into waste heat (which is why you need cooling devices such as fans in items like laptops). The CMOS sensor (and likely other components) is going to generate some heat when it's powered up which will be made worse if SteadyShot stabilisation (SSS) is also enabled (which will be moving the whole sensor array around) as that will also dump heat into the camera interior. If you are taking still images then it's unlikely these items will be in continuous use so overheating isn't particularly a problem then but video capture is a different matter. To reduce the effect of overheating it's recommended you disable SSS though even so you're likely to run into problems after maybe 20 to 30 minutes video capture (with SSS on I've heard it's going to be more like 10 minutes usage). As far as I know most DSLRs suffer with the same overheating problem with the sensor. Bear in mind that DSLRs with video capability are intended for video clip capture and typically you're looking at maybe 5 minutes duration (have you ever seen a Hollywood movie with an uninterrupted scene of more than a few minutes?) - they are not intended to replace conventional digital video cameras! I believe the camera will warn you about overheating anyway and will shutdown if it reaches a critical level to protect itself. Bear in mind too that heat in a sensor equals image noise so I would imagine image quality degrades to some degree during extended operation.
However, I don't quite know why it's a particular problem for video capture as in theory the sensor is active during extremely long exposures (typical situation would be shooting star trails at night with the shutter open for HOURS) though at least in this situation ambient temperatures will be often be rather low so the camera might be able to dissipate heat more easily.
