I've been reading the comments in Jeff Atwood's post about this problem:
Let's say, hypothetically speaking, you met someone who told you they had two children, and one of them is a girl. What are the odds that person has a boy and a girl?
The number of people who instinctively think it's 50% is high, as you'd expect. But the number of people who continue to insist that it's 50% after being shown that it isn't, is disturbing. What's most disturbing is that the target demographic for Atwood's blog is the software development community - programmers! Yikes.