I joined a private game of MW2 and I got one kill and instantly got to a 70. My friend said that it was because of a Jtagged console. Though I haven't heard any other cases of this but I'm now a first prestige 70 within a matter of a minute or so. So anyone on my friends list checks, they will see i'm a first prestige 70 with horrible stats.
Yup its possible, you can also unban a jtagged console with the keyvault from a console thats not banned, not really worth going on live with a modded console though. As you joined the game you should be fine and not get done for cheating, I joined a Gears 2 game hosted by KingBurns.. he had hacked horde mode from 5 to 9 players and made it so you got massive amounts of experience each round. everyone in the game ended up ranked 27 in the world on that map after playing 9 rounds.
Well, yeah I knew I wasn't gonna get banned. I'd be considered nothing compared to the ones with the Jtag console.