The phonegap installer was written for XCode 3.2 and doesn’t work with XCode 4. The reason is the new template folder layout for XCode 4. To fix this Shazron Abdullah wrote a small script that you can run after installing the 3.2 files.

Get it here:

http://blogs.nitobi.com/shazron/2011/03/16/creating-a-phonegap-project-from-the-command-line-for-xcode-4/

Edit: The blog seems to be down, but the article got mirrored at phonegap.com:

http://www.phonegap.com/2011/03/16/creating-a-phonegap-project-from-the-command-line-for-xcode-4/