Question :

MobileMe supports Find My iPhone on the iPhone 3G, but since iCloud is not supported on the iPhone 3G, what will happen to Find My iPhone on my iPhone 3G after switching to iCloud ?

Answer :

You cannot setup Find My iPhone on an iPhone 3G after you’ve switched from MobileMe to iCloud, but if you already had it working with MobileMe, it will keep on working after switching to iCloud (normally speaking)

Apple has posted the official info on this here :

http://support.apple.com/kb/HT4876?viewlocale=en_US&locale=en_US

It all boils down to this :

iOS-devices running iOS 4.1 or earlier are not visible in Find My iPhone on iCloud

iOS-devices running iOS 4 versions 4.2 and up should be visible in Find My iPhone on iCloud, but they’re not supported (and not recommended) by Apple

(note : iOS-devices running iOS 5 can run iCould natively, so they are fully supported)