Hi Sam - So far as the internet is concerned, ‘www.yourdomain.com’ and ‘yourdomain.com’ are completely different addresses and so getting both to work with your landing pages requires an extra step.
To use the naked domain (the one without the ‘www’ prefix), we recommend creating a ‘301 redirect’ to push visitors from ‘yourdomain.com’ to ‘www.yourdomain.com’ instead. You can read a bit more about setting this up at http://support.unbounce.com/entries/3….
Alternately, you can use the free service WWWizer (http://wwwizer.com) to handle the redirect for you. You simply point your root domain to their IP address with an A record. You can read more about their service at http://wwwizer.com/naked-domain-redirect.
If you have any further questions, please don’t hesitate to let me know!