When I saw this story on Yahoo! I didn't really know what to think. I'm a Child and Youth Studies major, so this is exactly the type of stuff we talked about in school.
Children are born with a sex (male or female parts, based on their chromosomes). Gender is much more of a social contruct, although there are definite genetic and biological differences between boys and girls that can affect their behaviour. But, if from the time you are born, you are bombarded with pink and dresses and dolls and all that other stuff, you are likely to think those are the things you are expected to play with/wear. On the other hand, if you're dressed like a boy, given sports balls and action figures, you're likely to think that those are the appropriate things to play with. There are certainly kids who break these stereotypes, but these "deviences" from expectations aren't always welcomed by parents, peers, and society.
I really do see no problem at this time with the parents not revealing the sex of the baby. However, I don't think it will last long, and I don't think the parents are expecting it to be a secret forever. Once Storm (which I do think is a horrible name, but to each their own) gets to be 2 or 3, he's going to start asking questions and deciding which gender he most identifies with. I'd be surprised if it didn't match his biological sex, but who knows? At that time, I think the parents need to be VERY honest with Storm and tell him that you were born as a boy/girl and this is what society expects from boys/girls, and that they as parents are supportive of him being who he chooses to be, but that society isn't always kind to those who deviate from traditional gender roles. I really think this can be explained in a kid-friendly way, and hope that it has already been explained to the older siblings, as it does appear that they have been encouraged to experiment with their gender.
That said, this is not a choice I would make and I do think it is a bit extreme. In my opinion, the more responsible decision would have been to say "our baby was born a boy/girl, but we want him to be free to identify with whichever gender he/she chooses, so please do not base gifts or behaviour toward our child on his/her sex." There are plenty of gender neutral toys and clothes out there that children can play with, and I see nothing wrong with having dolls and ponies and other traditional "girl stuff" available, even in a house full of boys (and vice versa for action figures and sports toys, etc with girls). I can only imagine that if these people are this open-minded, they must have plenty of similarly open-minded friends, as my guess is that the open-mindedness probably permeates the rest of their lives as well.
I also think that if keeping this "secret" seems to be negatively affecting Storm, or either of the older siblings, the parents should, in their best interest, reveal the child's sex.
The above is all assuming that the parents really are just very open-minded, perhaps unconventional people who truly feel they have their children's best interests at heart, not crazy people who just want publicity.