yes, but not all iphone 4 apps are available on the iphone 3gs/3g/2g becuase they are too high resolution apps, or need a front facing camera.
Yes!!! :D Japanese iPhone apps work on English iPhones, but if they haven't set it up for English as well as Japanese, all the text will be in Japanese.
Yes, all of them. However, some Iphone apps will not work with the ipod touch, so be sure to check when in the app store.
Well, the iPhone 3g has a camera so any apps that have to do with that won't work. Other than that no.
iPhone Apps, including games, work only on iPhone, iPod Touch and some work on the iPad.
Yes most of them are the same apps and will work on one or the other or both.
no, iphone apps are only made to work with iphones, not other phones...
only games can
Yes. Yes they do. But apps with location services won't be able to find your location.
Many apps will work on the iPhone, iPad and iPod Touch. Some are written just for the iPad while others only work on the iPhone and iPod Touch. Under the title in the App Store it will specify which devices the app will work on.
You buy the iPhone app
apple create the apps on iPhone