I never really do the social/multiplayer stuff on Steam (though once I clear my backlog I might start playing MP FPS games), but while looking around I stumbled up this Duke Nukem Forever global leaderboard.

I don't even know what Kill/Death is (aliens? multiplayer kills?), but assuming you can get one of those per second, that first place of a billion+ would still require around 32 years.

So can anyone explain what's this? Am I reading it wrong? Can you get bonus kills/death whatever which would explain this number? Or is it simply hacked? And if it's the latter does it mean that it's actually quite easy to do so? I'd have thought Steam would have taken measure to prevent this happening?

not sure, but seeing that #4 is only at 165,00, i'm gonna go out on a limb and say something isn't right with those first ones. unless he's died a billion times. I could see that. ;)
edit: and if that's PVP there no NO WAY that's legit.

Kill/death is exactly as it's written. A ratio of kills to the number of deaths. It means that number 1 gets over a billion kills to every death. Most likely it's multiplayer kills. It is probably a hacker because who can kill that many so fast?

Or it's kills divided by deaths, and the first guy's died 0.00001 times and made 10,362.736 kills, whereas the fourth one has died once and made 165,162 kills.

Yeah, I'm purposefully not making any sense with the numbers at the moment.

If you were posting this in the Steam forums then I think it's safe to assume that his account will be VAC banned by the morning.

this. he figured it out :)

seriously though, if that's pvp then his k/d ratio is stupid. If you have a k/d nearing 3 you are good. here is a k/d of the game I play: http://destinytracker.com/destiny/leaderboards/ps/kd

So if he broke a toenail while wiping out the entire population of Luxembourg...

Ah, so they take their scammers seriously over there?

Luxembourg only has roughly ten thousand inhabitants? But yes, something like that.

I would assume it is not legit, and it is not the only scoreboard with some ridiculous high scores on it. I think Valve simply does not care about people hacking high scores. As for difficulty, I would guess it involves hex editing the file that records your high score, so if you can find the right file, and the right value in that file, it would not be too hard.

Is DNF a VAC enabled game?

I'd assume it's this.

I quick estimate suggests that even if he was using in game hacks and he killed someone every second then he would have to have played for 31 years straight (and never died in that time) to achieve that score...

I think a number of leaderboards on Steam have hackers in them.

Steam are quite rigorous about removing cheats from VAC protected servers, yes. One of their most redeeming qualities, really.
I would have thought so. That's not to say that there aren't unprotected servers - they're the ones usually packed with the banned cheaters mind. 500,000. But I consider a stubbed toe to be more serious than a 0.00001 on the death-o-meter. It really bloody hurts!

No mention of VAC on their page. VAC only appears in VALVe's own games and others specifically designated. They are required to announce that a game is protected by VAC in the description. Dark Souls went through this where VALVe had to give refunds to people based on VAC being present before it was announced, then it was promptly removed.