According to tweet from MuscleNerd, iOS 5 now include popup unsecured call warning message to warn you when making an unsecure call as below.
The cellular network you are using is not encrypted. This call can be intercepted by unauthorized listeners.
You can choose to ignore this warning or end call.
This feature is more concerned about security issue which already revealed by Chris Paget during DEFCON 18 on July 30th – August 1, 2010 at the Riviera Hotel and Casino in Las Vegas. Chris Paget mentioned that cryptoscheme in GSM can be broken and intercept cellphone calls of your target within radio range.
Here is the video from DEFCON 18 to show you practical cellphone spying.