Sipwiz is correct the only way to do this properly is the HTTP and DNS hybrid approach. My registrar is a re-seller for Tucows and they offer root domain forwarding as a free value added service.
If your domain is blah.com they will ask you where you would like the domain forwarded to, and you type in www.blah.com. They assign the A record to their apache server and automaticly add blah.com as a DNS vhost. The vhost responds with an HTTP 302 error redirecting them to the proper URL. It's simple to script/setup and can be handled by low end would otherwise be scrapped hardware.
Run the following command for an example: curl -v eclecticengineers.com