Why does by iPhone App cannot load NSURL when linked against iPhone OS SDK 4.0 and run on iPhone OS 3.1?

Question

I have an iPhone App linked against iPhone SDK 4.0 but as deployment target I selected OS 3.1. When I start the application on my iPod touch running 3.1.3 I get an error that the class NSURL cannot be found:

dyld: Symbol not found: _OBJC_CLASS_$_NSURL
  Referenced from: /var/mobile/Applications/21ECAA8E-8777-4020-82F5-56C510D0AEAE/myTracks4iPhoneOS.app/myTracks4iPhoneOS
  Expected in: /System/Library/Frameworks/CoreFoundation.framework/CoreFoundation
 in /var/mobile/Applications/21ECAA8E-8777-4020-82F5-56C510D0AEAE/myTracks4iPhoneOS.app/myTracks4iPhoneOS
Data Formatters temporarily unavailable, will re-try after a 'continue'. (Not safe to call dlopen at this time.)
mi_cmd_stack_list_frames: Not enough frames in stack.
mi_cmd_stack_list_frames: Not enough frames in stack.

When I declare CoreFoundation as weak framework then I am able to start the App. But the class NSURL itself does not work.

Any idea?

Thanks, Dirk