If you're a .NET developer and you've worked with interop projects before, you've more than likely run into problems with HTTP 1.0 vs. HTTP 1.1 support.  .NET supports 1.1 by default, and as a result will break requests and responses up with the HTTP 100 continue header status.  Twitter apparently turned off HTTP 1.1 within the last week or two.  As a result my extenstion for my blog had to be modified since it was throwing as exception when trying to post to Twitter.

Adding System.Net.ServicePointManager.Expect100Continue = false; to your code before making the post will fix the problem.  In a web app though, you'll want to turn it back on afterwards.  Switching states isn't a good thing, and it is application wide, so be aware of the changes.