I want to install an AdHoc IPA file to an iPhone, but I don't want to back up the apps of iPhone into iTunes, because the iPhone is not mine, and there are too many apps in it. Is that possible?
You could also use Xcode (at least Xcode 4, haven't tried it with earlier versions):
Open the Organizer and select 'Applications' under your device in the Devices tab. Now use the Add button to add your ipa to the device.