The iOS InAppBrowser object has its own spinner which it adds to the webview automatically.
However, as the spinner is created as UIActivityIndicatorViewStyleWhite and the background of the webview is white you can't see it.
So I updated this like so:
self.spinner = [[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleGray];
Also, the x co-ordinate of the spinner's frame are outside the bounds of an iPhones view. To fix this I updated the frame to:
self.spinner.frame = CGRectMake(self.view.frame.size.width / 2 - 10, self.view.frame.size.height/ 2 - 10, 20, 20);
This should centre the spinner no matter what size of device you are using.
I have only tested this in portrait mode.
You may want to fork Cordova (I'm testing this with 3.4) and submit a patch or see if there is a "proper" way to do this.