They said that the iPhone will run on OS X. So i guess it is more or less a special version of Leopard, because it would be nonsense to release the iPhone with Tiger on it if 10.5 is so close, wouldn’t it?
I bet Leopard is good enough to go for the iPhone hardware configuration, just not yet ready for all the different Mac hardware configurations that are sitting on desks all around the world.
But who knows. If it is Leopard, it would be a pretty good chance for the freaks to finally get their hands on Leopard, since everyone who attended WWDC07 is pretty close to shit the pants when someone talks about sharing the WWDC07 beta.