PhoneGap application: "ERROR: Start Page at `www/index.html` was not found"

It's an incompatibility between PhoneGap and XCode 4. To resolve:

• right click on your project and choose "Add files to [project name]...";
• choose the www folder from your curent project's folder (it's included in there, but not added as a reference);
• when you select the folder, make sure you choose "Copy items into destination group's folder" as well as "Create folder references for any added folders".

Note: if you choose "Create groups for any added folders", the app will still fail at runtime.

For those who are as silly as me... I had forgotten to run cordova build.