Multiple domains pointing to same CI application

[eluser]peterbz[/eluser]
I am designing multiple location specific sites that are 99% similar. To be more specific, they are business directories that are city specific. I was wondering if it was a good idea to point all the different domains to the same CodeIgniter folder, and based on the URL, select different database that pertains to the specific city.

For example, I have domains:

http://www.NewYorkSite.com
http://www.DetroitSite.com
http://www.NewJerseySite.com

that would point to the same folder on the server.

Of course, the reason I'm doing this is to increase efficiency if I were to change something on functionality or add new feature. As I would be increasing these sites to over 10 locations in the future, it'd not be practical to update 10 different folders.

So the question is, first of all, is this a good idea? Also, if I were to go from this approach, how would I go about in changing CodeIgniter so that it will select different databases based on the URL. For example, if the URL is NewYorkSite.com, CI would select from the NewYork database. How would I go about doing that?