The latest edition of Jump Magazine reveals that Hope Estheim's associated summon will be Alexander. This means that almost all of the playable characters have had their summons revealed.
No scans are available yet, but there is also said to be new information about how chocobos will work in Final Fantasy XIII. We'll keep you posted when more information is known about this particular topic.
As in previously games, Alexander is likely to be Holy elemental.
Some new scans have surfaced on the internet which were picked up by FFShores, and they appear to show a new kind of monster that hasn't previously been seen.
Whether or not this is actually a new summon, nobody knows. However, it certainly does look feasible. One of the biggest questions probably revolves around whether or not the monster is actually an organic being, or a robot. While it has lights over its body, they could easily be part its armour.
It also doesn't look dissimilar to Diamond Weapon from Final Fantasy VII.