Ok, since after the release of QdQ this has become a known trick I got several questions about how to do it... here's a small FAQ:
Q: I've tried this for an hour and couldn't make it more than once! How often can you do it?
A: We were serious when we said it's the toughest jump in the game... (though that was before we discovered the new e1m4 jump, which is even tougher, and isn't even sure to make it's way into the next QdQ) In the start it took me several hours to succeed, but with the new throw method (which Matthias used first) it's easier, I can now do it about once every 15 minutes. So don't give up!
Q: How and where should I throw the grenade?
A: Aim a bit down towards the platform before throwing the grenade. You need to try and make it land close to you, far from the center of the platform. (Well it makes things easier) I know it bounces sometimes off, you can either restart or throw another one (but it's a waste of 0.6 seconds).
Q: When should I jump?
A: Well obviously you need to jump at the exact moment that the grenade explodes (or a very short while before or later, I don't think there is a big margin for error in this jump). Try to get used to jumping in the right moment... sure it'll take many tries to get it right. But there's another thing: make sure you're running up that tiny slope when you're jumping, it will give you the extra velocity that you need to make that jump. This is a must!
Q: Why do you fall from the left and not from the right, directly on the gold key?
A: That way the shambler doesn't see me (it was also Matthias' idea). Also you'll need to go back anyway to throw the grenade, unless you intend to throw it up (not recommended).
Q: Is it possible to throw the grenade already from the higher area?
A: Anthony will say yes, I say no :)
Ok, that's it for now...