I am on Snow Leopard and this one didn't quite work for me. But the following procedure worked:
- First added another account with admin privileges by ticking "Allow user to administer this computer" under Accounts, for example an account with username test
- Logged into the test account
- Launched Xcode, compiled and ran my iPhone project. All ok, no errors were thrown related to permissions
- Logged out of the test account
- Logged in with the another account having admin privileges
- Took away the admin priviliges from the test account by removing the tick from "Allow user to administer this computer" under Accounts
- Logged back into the test account
- Deleted the iPhone project directory and again checked out from the repository (in my case svn)
- Launched Xcode, compiled and ran the project. I didn't get any errors and the App ran well in the iPhone Simulator.