Perhaps we are seeing the emergence of a linguistic marker -- but I know little of linguistics.
Consider the top of the male last letter frequency distribution in 2006:
Last letter % of Males
Now look at the same distribution for female last letter frequency in 2006:
Last letter % of Females
(The last column counts each name on the 1000 list equally.) Note that the "n" frequency for boys is about the same order of magnitude as the "a" frequency for girls.
So perhaps we are culturally evolving to that letter becoming a marker of the child's sex.
There's also more here on Andrew Gelman's blog.