The patch will relocate the Lobby to a server created by a small group of Halo fans. The group's efforts were backed by Bungie, and it's the developer who will host the new server and release the patch "soon," no doubt much to the appreciation of the Halo community.
There's good news from Capcom too, with Lost Planet 3 migrating to a new service to retain its online multiplayer across all platforms. Similarly, Street Fighter 3: Third Strike Online Edition and Marvel vs. Capcom Origins have already migrated without issues. However, two of the studio's games will lose their online features when GameSpy's service shuts down on May 31, namely Age of Booty and Flock.
The two games join a growing list of games affected by the closure, including 50 from EA alone. Going by a removed Microsoft support article, Games for Windows Live's service may follow GameSpy out the door soon after, taking another whole chunk of games with it.