I had already installed the Mac OS X Developer Tools (a.k.a. XCode Tools) when I upgraded to Snow Leopard on my Mac Book Pro because I like to have current c compilers & such.  I never really used XCode, however, and the Tools didn’t include the iPhone SDK.  So I downloaded a more current one, Xcode 3.2.2 and iPhone SDK 3.2 (~2.3GB), which includes support for developing iPad apps as well as iPhone apps and includes a software emulator of the iPhone/iPad environment.  The iPad emulation doesn’t really fit on my MBPs 15 inch screen too well, but that’s OK.