I have had Leopard since the developer preview from WWDC 06, and through all the bugginess up till the last build released to developers I have gotten a feel for it.
That being said, there is no doubt that the phone runs on Leopard. Remember how they say it isn't a mobile OS, it IS OS X, sure there is some debate, but at the end of the day it's Leopard... they even said they stole members of the Leopard team to work on the iPhone.
This doesn't mean that Leopard's release = SDK. However it makes more sense. We all know how secretive Apple can be, and control freaks too. I am sure they could have released an SDK at WWDC this year, but they want to have some control over the applications. I installed AppTap like many people, and saw some impressive third party apps, however the majority of them were pretty useless.
I think once Apple figures out a way to control third party apps they will release an SDK... or they might just do what they did with the iPod and only let a few select companies develop apps/games.
In other news, I can't wait to hear more hype about Leopard, I love this phone, but give the new big cat some credit and let it share the spotlight for a little bit!
-Roy