This is a fairly bold move on the behalf of Red Hat, which recently hired Ben Skeggs, one of the Nouveau developers. The Nouveau developers have yet to issue a stable 2D or 3D driver release and that will not occur until their kernel mode-setting implementation has stabilized along with having proper Gallium3D support, but Fedora will incorporate this code anyways. More on the Nouveau driver's current status can be found in the Nouveau Companion 42 and their FOSDEM 2009 status update.
The latest Nouveau upstream code is currently in Fedora Rawhide, but the Gallium3D driver at this time won't be enabled since it's not yet in a usable state. The Nouveau driver is replacing the xf86-video-nv DDX driver, but this official open-source NVIDIA driver will remain installed and will be used as a fall-back if Nouveau doesn't recognize a chip or runs into other problems.
More details on the Nouveau driver being the default for NVIDIA hardware can be found on the Fedora Project Wiki. Ubuntu 9.04 is also getting the Nouveau driver, but it won't be enabled by default.
Bye bye to the xf86-video-nv driver. With its obfuscated code, lack of RandR 1.2 support, and many other feature handicaps, it will not be missed. In the future as more distributions start switching to a stabilized Nouveau driver, it will be interesting to see how NVIDIA handles the xf86-video-nv driver and whether they just kill it off or decide to play ball with the Nouveau developers.