Infinity Ward could almost foretell that someone would eventually hack Modern Warfare 2, with the lack of dedicated servers support, lack of console and even mods. So naturally someone has. They have managed to enable console and, by extension, the ability to join dedicated servers which were enabled by another modder. This then allows people to host and join dedicated servers, which is what the PC community has been begging for since the games release. The following video is an example of someone playing on a dedicated server which is running a mod, hence the low gravity and unlimited ammunition.
It was completely obvious that this would eventually happen to Modern Warfare 2 from the day that they announced no dedicated servers, which aggravated the whole PC community. While no one has been banned by Valve's Anti-Cheat (VAC) program yet, it is important to note that VAC often hands out bans weeks after it discovers a hack.