David,
I thought about putting a directory together as a project , so I sat down and put some thoughts on paper awhile back. Though I didn't have the programing ability and it seemed like the business model couldn't justify the expense /time for me to create. But I am more than willing to share a few of the ideas.
Doing some research I found a directory that I thought was pretty neat (MX is another hobby and passion of mine) and here is one style that utilizes ruby on rails technology.
http://www.motoxmap.com/map/index
A few thoughts....
I looked at it from the approach of creating a revenue stream. So many of my ideas stem from that.
First an foremost to make the directory good it has to be complete and comprehensive(IF there is not alot of detailers listed, then the directory won't be as relevant when someone is looking for a detailer). To do so, any detailer is able to add a free listing in one category (i.e. detailer, wash service, reconditioning , etc)including name and number.
The directory is search-able by Zip (radius) State , and then further broken down by category.
Upgrades would allow a detailer to add listings to additional categories, logo, website link or display ad on the page for an extra charge. I also thought about for those without a site, to be able to create a micro-site that allowed them to provide more detailed information. (of course for an additional fee).
The directory also provides information on detailing services built in to help those looking for a detailer learn about detailing and how to find a reputable detailer in their area. This info will also allow you to be more easily be spidered by search indexes to help drive traffic.
You have an extensive access to a large list of detailers and I am sure once the directory takes off a minimal fee for add ons can be easily justified. For example a link fee of $15 for the year seems to be a no brainer. One customer gained easily covers the cost.
Display advertising will allow you to prominately place detailers and their message/promotion. A vertical tower ad can be sold on impressions or some sort of action (a click).
I know some may dislike the idea of ad on charges, but if done correctly, the directory is a valuable asset for those looking and for detailers to gain customers/clients. This will help off set any cost /time invested.
Again these are just some thoughts.