The first alpha release is now available for Ubuntu flavors opting-in to do their first 16.04 "Xenial Xerus" development release.

Ubuntu itself continues to not participate in these development milestones in favor of early adopters, developers, and testers just using the latest daily ISOs. However, for others, their 16.04 Alpha 1 are out today. As of writing, Lubuntu, Ubuntu MATE, and Ubuntu Kylin are out with their 16.04 Alpha 1 releases while Xubuntu and Kubuntu are noticeably absent at the moment.

Ubuntu 16.04 "Xenial Xerus" will be officially released in April as the distribution's next Long-Term Support (LTS) release. More details via the mailing list announcement.