I have heard of this and researched it....There are ways to increase the chances of conceiving one sex or the other...
Timing when you "try"
can affect whether the baby will be a boy or a girl...the closer to ovulation, the greater the chance of a boy. The X sperm are slower, but can live longer, while the Y sperm are a little weaker but can swim faster.
Also, caffeine intake, the natural chemistry of the woman's body, sperm count, and other factors come into play.
This website covers it:
http://www.fertilinet.com/sexofbaby.htm