O Blog da RuslanY tem um ótimo post que descreve como fazer isso com o rewriter do IIS.
Vou citar sua dica # 3: -
Very often you may have one IIS web site that uses several different host names. The most common example is when a site can be accessed via http://www.yoursitename.com and via http://yoursitename.com. Or, perhaps, you have recently changed you domain name from oldsitename.com to newsitename.com and you want your visitors to use new domain name when bookmarking links to your site. A very simple redirect rule will take care of that: view plaincopy to clipboardprint?
<rule name="Canonical Host Name" stopProcessing="true">
<match url="(.*)" />
<conditions>
<add input="{HTTP_HOST}" negate="true" pattern="^ruslany\.net$" />
</conditions>
<action type="Redirect" url="http://ruslany.net/{R:1}"
redirectType="Permanent" />
</rule>
To see an example of how that works try browsing to http://www.ruslany.net/2008/10/aspnet-postbacks-and-url-rewriting/. You will see in the browser’s address bar that “www” is removed from the domain name.