From the Myst Online newsfeed:
Everything takes longer than expected these days. Cyan Worlds’ plans are to move MO:UL to open source, and we finally have some good news. We’ve taken a first step and started a MO:UL server, so the Ages of Uru are available again. We’ve opened all the Ages, and added most of the goodies in MO:UL. We’re referring to it as MO:ULagain—feel free to explore and enjoy.
If you haven’t experienced Myst yet, now you have no excuse.