I seem to recall some FoV changers are detected....And as all of them use the same method...All are detected... (Probably, though this may not be the case as some people use other hacks with it which may have caused the bans)
Anyways, I don't have MW3 on Steam but I usually use 90 as my FoV. The highest I'd go is 105. After that, the screen gets way too distorted for me to use.