Following in the footsteps of Final Fantasy X-2, Square Enix has announced a sequel to Final Fantasy XIII called Final Fantasy XIII-2. It will be released in Japan in 2011 and in the US and Europe "next winter" (so is that Summer 2011 for Japan and winter 2011 globally?)
Here's a leaked trailer. Be warned that it contains spoilers for Final Fantasy XIII.
Considering how well Final Fantasy XIII itself was received, I'm a bit surprised by the announcement of a sequel. I suppose this means I'll need to play the first one at some point.