I have created an extension for BlogEngine.NET which will update your Twitter status each time you add a post to your blog. The extension uses the Yedda C# Twitter library for interacting with Twitter. It also uses the TinyUrl API for shortening the URL of the post.
The source for the extension is here.
You will need to download the Yedda Twitter library separately and copy the DLL into the bin directory of your BlogEngine.NET website.
Comments and improvements are welcome.