Last week's Interviews with Innovators has Jon Udell talking with Jim Russell about the way the Web can be used to create social contexts among far-flung people who share some connection--in this case the fact that they all came from Pittsburgh.
I found this interesting because of a personal connection: my wife's from Pittsburgh--she's part of this diaspora. I lived in Pittsburgh for six months in 1983 (West Mifflin) and have been back many times. Scott Lemon, my co-host on the Technometria podcast is also from Pittsburgh.
Beyond the personal ties, I find the whole concept of regional diasporas fascinating. When I was building a company here in Utah and hiring was tight we specifically targeted the Utah diaspora. We'd tell our recruiter "find us someone with these skills living in the Bay area who wants to come home to the motherland." It worked surprisingly well.