It seems that the iPhone SDK for Snow Leopard (downloadable at /iphone and released in June 2009) installs outdated MobileDevice Extensions that are incompabile with the final release of OS X 10.6
I ran into a pesky problem after I installed the iPhone SDK (before that I installed XCode 3.2 from the Installer DVD) under Snow Leopard: My iPhone/iPod Touch was neither detected by Xcode nor by iTunes anymore.