>Do they wave back?

Yes they do! It's their way of saying "Hi back! Are you buying or holding?"

>In my hometown everyone you walk past will say "Hi, how 'ya doin?" even if they've never met you before.

Not in Miami. Because most of the time if you say "How 'ya doin" to someone, the response will be "Que?"

>And while driving on rural 2-lane Alabama/Georgia roads, most people will signal "howdy" by lifting one finger off the steering wheel as they pass by. (It's always the index finger unless they discover you're a yankee).

In South Dade, drivers will salute you by waving the barrel of an Uzi at you and if they don't like you, they do the same anyway.