You will have a hard time getting all of those answers considering that 3.2 is under an NDA. But I will try and answer what I can.
Apple does indeed advertise that most current App Store apps will run without any modifications on the iPad. The only gotcha is when those apps make use of features not present on the iPad. Apps that require a camera may not work, for example. This is no different than the App Store apps that will not run on iPod touches due to the same hardware limitations.
Apple also advertises that the latest beta of the SDK only works on Snow Leopard (10.6), so you would have to upgrade your operating system to even install the latest SDK. Considering the low price of Snow Leopard and its overall improvements over Leopard I upgraded the moment it came out, so consider the SDK's dependecy on 10.6 an added incentive to upgrade :-)
As for if the SDK includes simulators for iPhone and iPad... Considering that Apple hasn't publicly talked about that, I do not have any publicly available knowledge to back any of my statements with. So, under that context all I can say is that if you already have a developer account rush out and buy Snow Leopard and play around with the SDK. There is only the $30 Snow Leopard upgrade fee to lose.