Except that I can't figure out how to do it in routes. This is what I found.
In *config/routes/rb*, as the last route add:
map.connect '*path', :controller => 'application', :action => 'handle_unrecog'
In *app/controllers/application.rb*, put this:
#do something here, the path info is in the @params value
One of my uses for this is for a del.icio.us type user viewing. IE: _http://del.icio.us/[user]_ but having it only valid for users.
I'd do something like this:
uname = @params['path'] #first part of the path
user = User.find_by_login( uname )
return render :controller => "user", :action => "show", :id => user
return render :controller => "user", :action => "notfound"