Personally, I think it should be a required field when filling out the registration info. Without a city and/or ZIP Code, you won't get in the door.
Rumblefish360 and I were talking about this very thing on the phone a couple days ago. A map of all FABO members' locations would be a handy thing and let you easily find members who are close by. I'll dig around and see what I can find ... the Barracuda Owners Group had a Frapper.com map that was really cool.
Just my two cents.