Some chatter has been going around the iOS community about a possible feature in iOS 5 that would warn users of unsecured calls. The encryption used by GSM was cracked and a great presentation and demonstration (which I had the privilege of attending) were given about the crack at Defcon last year. The presentation is available on YouTube for free and is split up into four segments:
[youtube=http://www.youtube.com/watch?v=rXVHPNhsOzo]
[youtube=http://www.youtube.com/watch?v=Fo1OPoBS5Q8]
[youtube=http://www.youtube.com/watch?v=RXqQioV_bpo]
[youtube=http://www.youtube.com/watch?v=a4-KAvWUiDA]
Obviously this feature won’t be able to detect if a government agent at the phone company is listening into your phone call (this is why we need secure point-to-point communication capabilities on all phones) it would at least let you know if your phone call is being intercepted locally.