Lets Encrypt passed another major milestone by issuing our first certificate. The certificate is fully functional for clients with the ISRG root in their trust store. When we are cross signed, approximately a month from now, our certificates will work just about anywhere while our root propagates. We submitted initial applications to the root programs for Mozilla, Google, Microsoft, and Apple. Were working towards general availability over the next couple of months by issuing certificates to domains participating in our program.”]
Source: https://letsencrypt.org/2015/09/14/our-first-cert.html