A major factor to this is how linkbacks are written. For example, there is a difference how I provide linkbacks to my website using http://www.mywebsite.com, and http://mywebsite.com They both point to the same domain, but with or without an emphasis on the www.
Including (or not including) the WWW in the linkback is critical. On your web pages, you may also include the www in the copyright notice and throughout your page text, then add this to your keywords meta tag. One thing I should point out is to be consistant with how your promote your website, and how you write linkbacks.
Another important factor to remember is that subdomains will not use the WWW, nor will the WWW be shown in a search result that revelas a subdomain. To experiment, go to Live Search and search the term "web pages". There you will see (in the green text) the linkback URL. Take notice of the sponsored sites -- some use the WWW and others do not. Also take notice of sites that use a subdomain -- there will never be a WWW included in the linkback URL.
