Twitter announced on it's blog that it will not allow third-party advertisers to inject ads between tweets with the Twitter web development API. The company will be updating it's terms and conditions to reflect this restriction, which the company says is to produce the best user experience possible.
Part of this decision was to preserve the simplicity of the service. They want to keep the timeline attractive and not overload the timeline with clutter caused by ads. Another area of concern was that Twitter would ultimately be responsible for the maintenance and security of the content pushed by third-party advertisers.