1) They said they will stick to Austin City Limits, which means within these boundaries [1].
2) For those that followed up with how KC operated, Google runs their lines on electrical poles and all places the city power company has lines. This is one of the big things KC and Austin have in common: city owned electric utility. With that google can run all their lines much easier. So, if you get electricity from Austin Energy, you will likely be eligible to get Google Fiber at some point.
3) Beyond that, it will be up to people like us to advocate in our neighborhood to get people to sign up for the service. Whatever areas have a large # of people pledge to sign up will be more likely to get it. To me this means south and east Austin. But at the same time, north austin has a lot of tech employees from Dell, Freescale, PayPal, and other large companies (though many of those people may live in Round Rock).
[1] http://www.zipmap.net/Texas/Travis_County/Austin.htm
[1.1] (easier on the eyes map) http://www.maptechnica.com/us-city-boundary-map/city/Austin/...