Which Parts of the Galaxy Do Aliens Hang Out In?

Here at the Astrobiology Science Conference 2008 in Santa Clara, Charley Lineweaver, a Senior Fellow at the Planetary Science Institute is trying to figure out where in a galaxy aliens might live - the so-called "galactic habitable zone." Planetary habitable zones are well-known - for our sun Sol, we think it's roughly between Venus' orbit and the asteroid belt outside Mars' orbit. Galactic habitable zones are little tougher to pin down - as usual you've got to worry about having liquid water (or liquid something), but things get interesting when you consider the risk of getting blown up by a supernova.