I am also a member of Gen Y, having been born in 1987.
Most of my friends moved to the biggest city closest to where we grew up, which for me would be Pittsburgh. Of course there were some outliers who moved to Miami or Honolulu, but most stayed relatively close.
I personally see people in our generation moving away from small towns to bigger cities, mainly for economic reasons. Our generation has been fortunate enough to put many of it's people through college. The expectation was/is once you attain your bachelors degree, you will be able to land a job with a high enough income to substantiate the high expense of a college education. While there definitely are these types of jobs in smaller communities, they are few and far between and with such a large pool of qualified occupants, there are many losers.
This doesn't take into account all of the entertainment, cultural, etc aspects that will appeal to our generation as well.
One interesting thing that I was not aware of until I started investing. People from other areas of the country, actually receive more government money per month when they move from other parts to my area. It is a large incentive for lower income people to move to my area. So tracing the highest government payments by locale may be a good way to track the population movements.