I've been listening to Zandor's podcast and he reckons that a plant's sex is in the RNA and not the DNA, that is, it is not determined until it is a few weeks into the grow. He also gave a list of influencing factors which I can't remember right now. However some of them were:

Using a blue spectrum light (ie MH) in early veg should mean more females, whereas using a HPS would more influence males.

24hrs of continuous light in veg can influence more males, whereas a lesser amount of light (I believe he said 14 hours but unsure) will tip the balance towards females.

Check out the podcast anyway, it's pretty good. I tend to re-listen to them a lot because it's impossible to pick up everything on the first go (ps, not trying to kiss the dude's ass either, just my opinion).