Speed Governors and engine damage

Here's a question for you, generated in coversations with some drivers and stories passed on from others. As everyone knows, the American Trucking Association and assorted fleets are pushing to set the maximum speed for truck engines at 68 mph -- using engine governors pre-programmed at the factory to make sure trucks go no faster than 68 mph.

Now, what I am hearing is that some engine makers say equipping an over-the-road truck engine with a speed governor can damage it -- and that if you, the truck's owner, use a governor to restrict engine speed this way, it will void your engine warranty.

It's perplexing, to say the least, to hear this rumor -- for aren't slower speeds safer for the industry? And wouldn't restricting top-end speed actually improve the life cycle of an engine, since it there would create a 'ceiling' on how hard it would have to work? Let me know what you think.