Nitro is enabled in iOS for web apps added to the home screen, but not UIWebViews (aka, PhoneGap). While I don't think it's actively sinister, it doesn't seem like they're going out of their way to correct it, either.
The problem is that they can't allow programs to execute parts of the memory as instructions. This is needed for efficient JITs like the one in Nitro.
Apple is actively (Frequent commits) working on fixing this problem. The project is called Webkit2, and features a Javascript interpreter that runs in a separate process, and thus is able to safely execute Javascript on iOS. This model, btw., is inspired by the multi-process architecture found in other major browsers.
