I have downloaded the iOS 5 SDK and found that ARC is a great feature of the new Apple compiler. For the time being, many third party frameworks don't support ARC. Could I use ARC for my new code and keep the current retain/release code unchanged? The ARC converter doesn't work here, because some frameworks, such as JSONKit, cannot be converted to ARC by using the converter.
Edit:
The answer is to add -fno-objc-arc
to the compiler flags for the files you don't want ARC. In Xcode 4, you can do this under your target -> Build Phases -> Compile Sources.
It is not very intuitive how to disable ARC on MULTIPLE files, for a while I was do it one by one until a figured out how to do that.