Simple Redirect - This doesn't worry about social referral tracking, providing a different URL for mobile etc. It's just meant to be easier to understand and implement.

fbcanvas.liquid

{% plugin rawtext desktop_redirect_url %}

<script type="text/javascript">
if (!document.body.className.match('page_preview')) {
  // Redirects as soon as possible
  setInterval( function () {
    top.location.replace({{ desktop_redirect_url | json }});
  },250);
}
</script>

Instructions

1. Download your current template from the My Templates section of of the admin.
2. Unzip the file.
3. Create a new file in the template folder called fbcanvas.liquid. The content of your folder should now include something like 'default.liquid', 'fbcanvas.liquid' and a folder called 'assets'.
4. Paste the code above into the fbcanvas.liquid and save.
5. Select the liquid files and assets folder.
6. Right-click and create a zip.
7. Upload the zip you created as new version of your template.
8. Update or create your page.
9. Configure the "Desktop Redirect Url" plugin with the the full URL to the page you want people to view if using a desktop device (e.g. the URL to a fan page tab).

That's it!