I've suspected people of cheating but not until last night did i see one that looked like I had any proof with the kill cam, but if he was cheating, he sucked at cheating. 
We were on Favela. I watched as the guy was coming to the same corner i was about to come around. all of a sudden he stops and brings up his weapon and then blasts me as i come around the corner. I was using cold blooded perk and hadn't fired my weapon. He wasn't using a heartbeat sensor so there was no way he could have known I was there.
		
		
	 
That happens to be quite a bit.  Although if you weren't running Ninja he could've heard your footsteps.  Especially if he's a serious gamer and is using directional headphones like I do - very easy to hear footsteps and tell where they're coming from.
I was in a search and deystroy server yesterday and for the first time saw irrefutable proof of a hacker.  In S&D there are no respawns, so when you die you can observe your teammates.  One of my teammates had to have every conceivable hack on.  He'd hold down the fire button and his sights would keep hopping from enemy to enemy, headshotting them all.  It was embarassing and we all told him to get a life and left the server, but then IW's wonderful match-making system just keeps dumping people into his game so he has more people to kill.
If you witness an obvious hacker PLEASE report it.  Hit shit+tab to bring up steam, then click players, then "previous games" and find the player in your list of people you've recently played with.  Click on their "steam ID page" and then in the upper right look for "report misconduct".  If enough people do it maybe they can look into it and ban the jackasses.  Although maybe that's wishful thinking.
Also, I agree, if you are of the type "can't beat them so thinking about joining them" you're no better than the original hackers.  You do that and it only leads to more and more people hacking and it's a loss for all involved.