WWWizer is pretty cool, but I'd rather roll my own on Heroku. I have a Heroku "app" that Heroku thinks is PHP, but it just contains a .htaccess file with some RewriteRule directives to 301 http://example.com to http://www.example.com. I then do the real hosting elsewhere. :)
Personally I prefer [1] using a subdomain over the bare domain. If your registrar doesn't support www redirection you could always use WWWizer [2].
[1] http://vvv.tobiassjosten.net/internet/using-www-for-your-dom... [2] http://wwwizer.com/