C# – Twitter API + OAuth: Can’t send status updates, getting 401

I'm trying to use Twitter's API and OAuth to send status updates (new Tweets). I am using Shannon Whitley .NET code example http://www.voiceoftech.com/swhitley/?p=681 (as recommended on the Twitter API docs). I can read (GET) using OAuth just fine, however when I try to send a status update via http ://twitter.com/statuses/update.xml (using a POST), it returns a 401 with the following XML:

<?xml version="1.0" encoding="UTF-8"?>
<hash>
  <request>/statuses/update.xml</request>
  <error>Read-only application cannot POST</error>
</hash>

I swear I have set up my application to use read and write, the authorise page at Twitter (http ://twitter.com/oauth/authorize) even says "The application TweeVerbs.com (Development) by would like the ability to access and update your data on Twitter."

Yet it's still saying "Read-only application cannot POST". WTF!?

I have googled this error message until I was blue in the face. I found somewhere that said to add the querystring paremeter oauth_access_type=write to the redirect URL which goes to the Twitter authorise page which I have done, but it still gives me a 401.

If it helps, here is the data that is getting sent back and firth as per the OAuth workflow:

Request Authorise Token:

http ://twitter.com/oauth/request_token?oauth_consumer_key=tViV8vAt4cqSKbGdPGWT7Q&oauth_nonce=2790042&oauth_signature_method=HMAC-SHA1&oauth_timestamp=1244567068&oauth_version=1.0&oauth_signature=KzxcXN%2bQ0AJoAJ%2flQfzs8SLjC%2fQ%3d

Generated Authorize Redirect URL:

http ://twitter.com/oauth/authorize?oauth_token=EpyBg3nJGOmtmBjRUAsqqaGHARb2F2F2VcccqHkwio&oauth_access_type=write

Authorise Screen Message: "The application TweeVerbs.com (Development) by would like the ability to access and update your data on Twitter. This application plans to use Twitter for logging you in in the future. Sign out if you want to connect to an account other than Sironfoot."

Get Access Token:

http ://twitter.com/oauth/access_token?oauth_consumer_key=tViV8vAt4cqSKbGdPGWT7Q&oauth_nonce=2016804&oauth_signature_method=HMAC-SHA1&oauth_timestamp=1244567166&oauth_token=EpyBg3nJGOmtmBjRUAsqqaGHARb2F2F2VcccqHkwio&oauth_version=1.0&oauth_signature=%2bEVQUxUPLT%2b%2bkfaG0Vq1YJZXcAw%3d

Status Update API call:
URL – http ://twitter.com/statuses/update.xml
POST data – oauth_consumer_key=tViV8vAt4cqSKbGdPGWT7Q&oauth_nonce=5707692&oauth_signature_method=HMAC-SHA1&oauth_timestamp=1244567268&oauth_token=19130957-nb89DjZhjCAzcbHUa96yRWHqlQFQIJ0AKyXpqnHt1&oauth_version=1.0&status=HelloWorld&oauth_signature=WqA%2bWY0IxveeSJ7G3Ewy3whh1sE%3d