I just finished up the beta of this Windows Live Writer plugin. It automatically uses bit.ly to shorten your post URL using your bit.ly account so you can track it later, and then sends a tweet.
This supports the new(ish) Twitter OAuth workflow, so you won’t get any more Access Denied junk from outdated plugins.
Let me know if you have any questions about this.
Bitly Tweeter is available on CodePlex under the MIT license.
There is a blog post in the works on how I built it, in case you are interested in extending Windows Live Writer!