I use restful_authentication (RA), but I think there are other more popular Plugins now. I think this would work with any Plugin.
If you ship your site to someone else who then is responsible for initializing it and setting up the users table, you could just ship it with an empty users table. Then you could do this in the application_controller:
render :action=>:index #display your normal index (main) page
redirect_to (:controller=>:users, :action=>:new, :type=>:admin)
In the case of RA, which doesn't come out of the box (or didn't 3+ years ago) with roles, you could just extend the users table to use Single Table Inheritance, i.e add a type attribute to the User model.
Make sure that for normal users you always pass :type=>:user, and the only place where :type is set to :admin, is when the user table is Empty.
From then on, you check if a signed in user has admin privileges, you could do
if current_user.class == Admin
current_user is the RA method for accessing the currently logged on user, other authentication plugigs will have the same or a similar method. Other Plugins may support Roles, so instead of hacking RA to use STI, you would utilize Roles in the plugin, but the concept is pretty much the same.
Joe got a job, on the day shift, at the Utility Muffin Research Kitchen, arrogantly twisting the sterile canvas snout of a fully charged icing anointment utensil.