This. Using a heartbeat monitor is a perfectly legitimate tactic.
Is it just me, or is COD steadily becoming the new COUNTER-STRIKE? I've seen countless examples of where players jealously guard their skills, of where people rage-quit and accuse others of being noobs simply because they get killed. It seems some players would like nothing more than to see most of the weapons removed from the game and everyone forced to play with a select set of weapons (that usually favour their own strengths) because it would be a "true" representation of peoples' ability. If I'm sitting on Afghan with a sniper rifle and I take you out with a headshot because you broke cover, that doesn't make me a noob. If I'm running into a room and I can't see what's around the corner and so I sent a grenade from the launcher before me and it kills you, that doesn't make me a noob. And yet in almost every game I play, someone is screaming and being abusive about it. There was a guy camped out in the substation on Derail once, and I managed to circle around behind him via a largely-unused part of the map, jumped over the claymore he had waiting and knifed him from behind. Did I get a "Whoa, that was a good stealth kill you had going"? No. I got "****ing noob! Someone kick this ****ing ****!", and all because I showed some guy that he wasn't nearly as good as he thought he was. People completely miss the fact that they're totally predicatable; everyone has a preferred route through each stage, even if they claim otherwise. It's the people who don't know the maps that you have to be wary of because they've got the element of surprise.
I think this attitude is really killing the multiplayer. I understand that some gamers feel the learning curve in MW2 is non-existent, and I think their complaints here are somewhat justified (although they could always go in for a Mercenary game ... this never seems to occur to them, though). But their behaviour isn't. If ever you join a COD-related message board or try to edit the Wikia, you're met by a barrage of hostile posters and ostracised if you don't value the same skills as they do. I have my preferences for a certain set-up, yet I get accused of cheating because I use Painkiller and get a health boost for ten seconds every third death. The wider COD community is filled with obsessive and openly-hostile gamers who are very unfriendly, accuse everyone who beats them of cheating and cannot believe that they are anything less than the best gamer ever. I'm sure this isn't what Tryarch or Infinity Ward intended - but it's what they're getting.