I doubt it's 40.
40 literally takes around 5-6 hours to get to, and is basically a tutorial of the game.
Not to mention that it's when you get your sub-wep, and pretty much when the game begins.
I've no clue why they'd limit it there, 50 would make more sense.
In any case, no where was it stated that the level cap is 40, and somehow someone spread a rumor that 40 is the level cap. Just keep in mind that this wasn't officially stated anywhere.
black's got the correct info from the F.A.Q, but also keep in mind, all the other servers have lowered their level cap from 75, after having them at 99, so we'll be seeing that here soon.