Besides a bunch of fixed issues on Windows XP and Vista there's also a new feature called MultiView:
Catalyst 8.1 introduces MultiView support. This feature provides for hardware accelerated OpenGL rendering across multiple graphics adapters. MultiView will provide hardware accelerated 3D rendering in a system containing multiple graphics cards on an extended desktop arrangement. This feature will allow for the rendering performance and additional frame buffer resources to be evenly shared with the second and third graphics adapters. This allows for a 3D application to have the same performance running on a secondary or third display device as if it were running on the primary display device. This feature will be supported under Windows XP (32 and 64 bit versions), Windows Vista (32 and 64 bit versions), and the Linux operating system.I'm not sure if the Catalyst 8.1 drivers support CrossFire X, some people who tried the leaked Catalyst 8.1 beta drivers say it works but the release notes of the official Catalyst 8.1 drivers say nothing about it so I suppose support for CrossFire X is still not ready.