Leaks on iPhone, but not in Simulator, involving external libraries?

Question

I need help with pinpointing source of leaks that all originate from external libraries, so I can't figure out if it's anything I'm doing. Strangely, on the simulator there are no leaks!

Some of the Leaked Objects / Responsible libraries / ResponsibleFrames that show up are these:

UIImageView      iAd                -[ADBrandingFrame initWithFrame:]
CALayer          UIKit              -[UIView _createLayerWithFrame:]
NSCFString       Foundation         -[NSCFString copyWithZone:]
GeneralBlock-88  QuartzCore        mem_alloc
GeneralBlock-32  ImageIO           ImageIO_Malloc
GeneralBlock-32  GraphicsServices  GSFontGetFullName
GeneralBlock-16  GraphicsServices  GSFontGetFamilyName
GeneralBlock-16  WebCore           RunWebThread(void*)
GeneralBlock-16  JavaScriptCore    WTF::initializeMainThreadPlatform()

WebCore and JavaScriptCore leaks are small leaks but among the first to show up. The last one produces a backtrace like this one:

 0 libSystem.B.dylib thread_assign_default
 1 libSystem.B.dylib _pthread_start
 2 WebCore RunWebThread(void*)
 3 CoreFoundation CFRunLoopRunInMode
 4 CoreFoundation CFRunLoopRunSpecific
 5 CoreFoundation __CFRunLoopRun
 6 libSystem.B.dylib malloc

Any ideas out there as to what the problem may be? I'd have a hard time believing that the leaks are all in the external libraries, but on the other hand no leaks show up in the Simulator. Is there a bug in the Leaks application? Should I not worry about these leaks?

Thanks for your help.