I'm going crazy only talking about this on Japanese forums, and love to hear the opinions of all you other guys who beat it or are nearing the end.
 
First - The Bridge at the end of Chapter 11, everyone's saying it's the same bridge in the Versus XIII trailer but we're seeing the other side of it.  It makes sense, and it gives a good hunch something bad happens in Versus XIII that caused people to live up in the sky.  
 
Second - If you read the novel or back story (in the menu), it talks about how humans are still living on the planet and fear the people in the Sky.  This is also where Vanila grew up, as she isn't a native human who lived on that sky planet.  So, more people are saying it even strongly confirms Versus XIII is more tied to this.
 
Third - At the end, the sky world dies along with everyone in it.  It crystalizes, and the big hunches are that maybe Agito takes place AFTER FF13 and not the first game in the series.  Again, this would make perfect sense as the techonoloy isn't as advace from looking at the Agito XIII trailer and if you read the Japanese site for the game, it talks about how the religion of FF13 is still going.
 
Fourth - Religion.  We don't know if the people in Versus XIII have the same faith as those in Agito and FF13.  However, if Agito XIII has the same religion and Versus XIII doesn't, it will be a strong confermation that Versus XIII is the start and Agito will be the end.
 
So we got two main possibilities on how this Series will go with timeline:
 
Final Fantasy Agito XIII -> Final Fantasy Versus XIII ->  Final Fantasy XIII
 
Final Fantasy Versus XIII ->  Final Fantasy XIII -> Final Fantasy Agito XIII
 
Both easily make sense.  You guys have any other theories that we could share on forums?  I can't believe we'll have to wait until late 2010 to get our hands on Agito XIII for the next third of the puzzle.  >.<