The client is available and ready for testing today at https://github.com/letsencrypt/letsencrypt. However, there are still lots of changes that need to go in before we launch. We have been in touch with people who maintain packages for various distributions and plan to coordinate releases with them, so packages will be available as soon as possible. I think we can’t guarantee that distribution packages will be available on launch day, especially for Extended Support Releases of distributions, but we are going to do the best we can on that front.