Update:

I figured out why I was getting the grant type error. I wasn't un-protecting the refresh token in my test code that is trying to refresh the token. So now that I have that fixed, I can get a response from SF with a new access token.

However, all responses are still in JSON, regardless of which response format I request. The endpoint seems to no-longer honor the HTTP Accept header, nor the format parameter.

If I use the following code, I get back JSON even when I ask for URL-encoded or XML:

HttpWebRequest webRequest = null;
HttpWebResponse webResponse = null;
try
{
    webRequest = (HttpWebRequest)WebRequest.Create(SALESFORCE_REFRESH_TOKEN_URL);
    webRequest.Method = "POST";
    webRequest.ContentType = "application/x-www-form-urlencoded";
    //webRequest.Accept = "application/x-www-form-urlencoded";
    using (StreamWriter writer = new StreamWriter(webRequest.GetRequestStream()))
    {
        System.Text.StringBuilder sb = new System.Text.StringBuilder();
        sb.AppendFormat("grant_type={0}", HttpUtility.UrlEncode("refresh_token"));
        sb.AppendFormat("&refresh_token={0}", HttpUtility.UrlEncode(token.RefreshToken));
        sb.AppendFormat("&client_id={0}", HttpUtility.UrlEncode(token.ConsumerKey));
        sb.AppendFormat("&format={0}", HttpUtility.UrlEncode("xml"));

        writer.WriteLine(sb.ToString());
    }

    webResponse = (HttpWebResponse) webRequest.GetResponse();
    using (StreamReader reader = new StreamReader(webResponse.GetResponseStream()))
    {
        String responseBody = reader.ReadToEnd();
        ...
    }
catch (WebException err)
{
    ...