I’m trying to compile an old project that I changed the UI a bit, but I’ve updated to Phonegap 1.0 and Xcode 4, so I’m getting this errors:

What should I do?
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It looks like you now have ARC (Automatic Reference Counting) set on in the project. To use ARC your code will have to change, one of the changes is the way the autoreleasepool is created/drained.
Other things that are not available are retain. release and autorelease, the compiler generates all of these behind the scenes as necessary. This all works iff all of the Obj-C naming conventions have been followed.
Xcode has refactoring help to convert a project. However this many not be appropriate in your case, if that is true turn it off in the project/target build settings:
ARC can be disabled on a file-by-file basis:
There is no problem mixing ARC compatible files with non-ARC files. This can help bring legacy projects to ARC. ARC is the future.