Code used by the WindowsPhoneOAuth sample.

AssociationUriMapper.cs

class AssociationUriMapper : UriMapperBase
{
    public override Uri MapUri(Uri uri)
    {
        var tempUri = System.Net.HttpUtility.UrlDecode(uri.ToString());

        if (tempUri.Contains("sfdc://success"))
        {
            var querystring = tempUri.Substring(tempUri.IndexOf("#") + 1);
            var split = querystring.Split('&');
            var accessToken = split[0].Split('=')[1];
            var instanceUrl = split[2].Split('=')[1];

            var url = string.Format("/MainPage.xaml?AccessToken={0}&InstanceUrl={1}", accessToken, instanceUrl);

            return new Uri(url, UriKind.Relative);
        }

        return uri;
    }
}

CreateAndBrowseAuthURL.cs

var url =
    Common.FormatAuthUrl(
        AuthorizationEndpointUrl,
        ResponseTypes.Token,
        ConsumerKey,
        CallbackUrl,
        DisplayTypes.Touch);

AuthBrowser.Navigate(new Uri(url));

InitializePhoneApplication.cs

private void InitializePhoneApplication()
{
   ...

    RootFrame.UriMapper = new AssociationUriMapper();
}

LoadAccounts.cs

var client = new ForceClient(InstanceUrl, AccessToken, ApiVersion);
var accounts = await client.Query<Account>("SELECT id, name, description FROM Account");

OrganizationsList.ItemsSource = accounts;

WMAppManifest.xml

<Extensions>
  <Protocol Name="sfdc" NavUriFragment="encodedLaunchUri=%s" TaskID="_default" />
</Extensions>