iOS 5.1 compass app on iPhone 4 won't show true north heading

I have an iPhone 4 running iOS 5.1.  The Compass app that comes with the OS will  allow me to display a heading in magnetic north but not in true north.  The true north option  is grayed out.  The magnetic heading displayed is accurate.  I have location services turned on and the app displays my correct LAT & LON.  I don't use this app very often but I'm sure both magnetic and true north options working before.  It may have worked with iOS 5.0 and stopped working when I upgraded to 5.1.  I tried powering the phone off and back on several times.  I haven't tried restoring it with iTunes.  Any thoughts?