I hear that FF XIII is planned and/or estimated to be as huge as Final Fantasy VII and/or Final Fantasy I or something. Between FF VI and FF VII, Square made the Final Fantasy VI: Interactive CG Game, that took some months of development. Final Fantasy Versus XIII is a seperate game from Final Fantasy XIII, they just take place in the same universe with the same mythology. The development teams for each game are different, except both have the same character designer, and probably the same scenario writer. The rumor about FF Versus XIII was declared false by Square Enix right after it came out, interesting rumor. Expect FF XIII to be out in Fall 2009 in Usa and one year later for FF Versus XIII. This is great!

Oh the various reasons for the delay in releasing Final Fantasy XIII(six year development cycle expected roughly)
1. They started working on it in late 2003 or early 2004 for PS 2, but after making the FF VII PS 3 Technical Demo for E3 in late Spring of 2005, they restarted development, using PS 3, and only some of the work carried over, most had to be restarted.
2. They are finishing up working on the Crystal Tools(White Engine) development tools kit, this took a bit of time.
3. I hear that Playstation 3 is difficult to program for, this may cause a delay.

Anyways, now I'm wondering if after Final Fantasy XIII is out, will Square Enix want to develop FF XIV for PS 3 or Xbox 360, we'll have to see about this.