Square Enix has registered a domain called FinalFanasyXIII-3.com. As noted by Joystiq, Corporation Service Company was who signed the domain, but its done domain registrations before for the publisher. XIII-2 was first uncovered before its formal announcement via a domain registration back in January. You probably should’t read too much into this, though: it’s probably a bit of safekeeping from domain squatters by Square.
FFXIII-2 launches in January for PS3 and 360.