In an update to a blog post, the Diaspora team have clarified that the 15th September release date they revealed earlier was only the release date for the source code. The consumer facing alpha will be released in October.
This is what they wrote:
To clarify, September 15 will be our open-source developer release. At that time, we will open up our github repository, publish our roadmap, and shift our development style to be more community oriented. We intend on launching a consumer facing alpha in October.
This is not a setback or any kind of pushing back of the release date. When they announced September 15 as the release date for Diaspora, it has lead to some confusion that users are actually going to be able to use Diaspora from that day. Just to clarify on the situation, they have posted this message, before people gets too excited with the September 15 date.
If you are a developer, though, you can start working on the code from September 15. Users will have to wait for the alpha in October.