How is it cheating? You have guns, and a radar showing you where people are, and sniper-rifles and communication.
I don't even think it's a cheap strategy from that point of view.
Mind you, I've only played MW2 for about an hour (at a mates house, I don't even have a console from this generation *sniff*), and was a happy camper in some matches - the level with the house in the middle of the woods, I found a great gap in the picket fence to snipe my friend (who is an amazing sniper) from. It was great - 1 guy with a machine-gun and 1 with a Sniper on each team. The second map was on some city level, with a car parked at the end of a narrow street. The enemy pair had to come down one street to the left (which my friend was defending with a sniper rifle) or down my way towards a car, and I was using a sniper rifle to pick people off and the bonnet of the car for cover. I did get taken down to lowest health by a grenade blast, but still didn't get dug-out. Another example would be on yet another city level, hiding around corners and doorframes with a shotgun and taking people out as they ran through (using the radar as guidance).
Are any of these cheating? I don't think so. Were they a poor strategy? I don't think so - I was playing against two people who owned a PS3 and the MW2 game, the third was the girlfriend of the PS3 owner and had played a couple of times. Ie, I was a noob and they were very experienced, yet some simple "tactics" (sneaking, cover and lanes-of-fire) meant I could hold my own against them occasionally.
I dunno, maybe most people play with the radar off, or maybe I'm too used to Halo/Halo2 and other games where you rely on the HUD to know what's going on most of the time.
So yeah, I don't think knifing is a cheat/noob-strategy, nor camping for that matter. People just don't like it when you best them at something (ie I found that despite being an amazing Halo sniper I really suck at it on MW2, but when I start sniping with the heavy machine-gun I'm pretty good - I found it a really surprising weapon for me in that regard - stopping power, range, some zoom and a helluva lot of firepower up-close).
But this is all coming from an utter MW2 noob.