+1  

OMG, Seriously, people are just randomly changing numbers without knowing WHY those numbers should be what they are.

15 seconds isn't going to be enough for blocks to get from one side of the network to the other. A couple of miners are going to modify their clients to ignore other chains, and just mine everything themselves. With blocks only 15 seconds apart, it's going to be almost impossible to avoid multiple, simultaneous, attacks on the coin.