I disagree with what jcm says about this. It's not a terrible idea at all and is used in production by the two biggest social networks Facebook and MySpace.
The route to match http://example.com/username
would look like this:
map.connect ':username', :controller => 'users', :action => 'show'
If you want to go the subdomain route and map profiles to a URL like http://username.example.com/
, I recommend using the SubdomainFu plugin and the resulting route would look like:
map.root :controller => 'users', :action => 'show' , :conditions => {:subdomain => /.+/}
These broad, catch all routes should be defined last in routes.rb, so that they are of lowest priority, and more specific routes will match first.
I also recommend using a validation in your User model to eliminate the possibility of a user choosing a username that will collide with current and future routes:
class User < ActiveRecord::Base
validates_exclusion_of :username, :in => %w( messages posts blog forum admin profile )
…
end