Google does not select cities / agencies to include in Google Transit -- agencies decide to participate and supply Google with data in the Google Transit Feed Format (GTFS). The best way to get your local agency included in Google Transit is to write a letter to your local agency.
From the Google Transit FAQ:
http://www.google.com/intl/en_us/help/faq_transit.html#adding_an_agency
More information on GTFS, the Google Transit Feed Specification:
http://code.google.com/transit/spec/transit_feed_specification.htm
