Www_redirect is a project mainly written in COFFEESCRIPT and RUBY, it's free.
DNS servers don't allow an A record to be mapped to a CNAME. When you are using google appengine or dotcloud for
hosting which doesn't give you an IP address but a CNAME, you can map www.yourdomain.com to the CNAME given by your
hosting provider and redirect your A record to this running code.
For example, you can run either of www_redirect.coffee or www_redirect.ru on your VPS with static ip say
227.18.10.190. Then you can set yourdomain.com A 227.18.10.190:80(assuming the service is running on port 80)and
the service will redirect it to www.yourdomain.com.
This is just one of the ways to redirect naked domains to www subdomain. You can redirect using your webserver as well.