Shattered Moon has actually posted a news update to the Shattered Moon website:

Classic Ultima Online is once again available to play, thanks to server hosting by David Baity ofwww.megawars.net. Please visit the download page for the RUM Portal utility used to download, update, and launch the game.

The CUO client requires an official installation of Ultima IV Quest of the Avatar, so be sure to get your free copy from gog.com.

The download page further explains:

The CUO client requires an official installation of Ultima IV Quest of the Avatar, so be sure to get your free copy from gog.com. For the full CUO experience, you may also be interested in Ultima I, II, and III, also available at gog.com for a modest fee.

With this release, Shattered Moon has done away with the standalone client for Classic Ultima Online, and has replaced it with something called the RUM Portal. This utility — hosted at both the Shattered Moon website and here at the CUO project entry — is used to download, launch, and patch the Classic Ultima Online client, as well as any other games that might eventually be made using Shattered Moon’s RUM framework.

So if the previous method of installing and getting in to Classic Ultima Online has been a hurdle for you, this should hopefully make for a smoother experience.