It hasn't happened quickly. Any release takes time and effort, I know very well, and Cyan has been focussing on the projects it needed to survive.
But today the announcement came through:
Today we are announcing that the sources for the MOULA client engine and development tools (CyanWorlds.com Engine) will be made available as open source. At the same time, MOSS which is a MOULA server replacement (written by a'moaca' and cjkelly) will also be released. Both open source projects will be hosted on OpenUru.org.
The goal of the open source CyanWorlds.com Engine and the MOSS server is to provide a "playground" where new writers can learn their craft, and new maintainers can inspect it, and new cartographers can map it. The Cyan Worlds MOULA servers will continue to provide a (relatively) safe environment for the D'ni faithful to mingle and share.
(-- from a letter from Rand Miller, posted April 6 on the Myst web forums)
As you see, this is a joint effort: Cyan's client code, Cyan's modelling tools (3DSMax plugins), and a compatible server implemented (from scratch) by members of the fan community. All are available now, although you currently have to register for the download. I expect mirror repositories will pop up by tomorrow. (The server is GPL3; I haven't seen a citation on Cyan's license yet.)
If you can't tell by my hasty typing, I'm utterly jazzed about this. I wish I could spend a month or six learning the modelling I'd need to start firing up my own pieces of the Myst multiverse. But I have my own projects spread out before me, as you know.
Nonetheless, I am about to jump into the game -- which I now have to specify as Cyan's game, which will remain as the core of the Ages of Myst. I'll be in the pub, toasting with the gang.