error MT5309: when building for device, but not on simulator

Hi All-

I have a strange issue. I am getting the following error:

error MT5309: Native linking error: final section layout:
error MT5309: Native linking error: __TEXT/__text addr=0x00009000, size=0x01085C9C, fileOffset=0x00005000, type=1
error MT5309: Native linking error: __TEXT/__picsymbolstub4 addr=0x0108EC9C, size=0x00006D90, fileOffset=0x0108AC9C,     type=27
error MT5309: Native linking error: __TEXT/__stub_helper addr=0x01095A2C, size=0x00001B90, fileOffset=0x01091A2C, type=31
error MT5309: Native linking error: __TEXT/__objc_methname addr=0x010975BC, size=0x0000777F, fileOffset=0x010935BC, type=13
...

when building for a device (Debug > iPhone), but NOT when building for the simulator.

I have tracked down what seems to be triggering the linking error, but I don't know how to fix it. I have a xib (Foo.xib). If I add a button (FooButton) to Foo.xib, I'm ok, FooButton shows up on the device and the simulator. If I try to create an IBOutlet to said button...

@property (nonatomic, retain) IBOutlet UIButton *FooButton;

...I get the linking error when building for a device, but not the simulator. The iOS Build settings in my Project Options are the same. (I have many other buttons with outlets in Foo.xib, all working fine). Any ideas?

My vital stats: Xamarin Studio Business Edition 4.2.3 (build 60) Xamarin.iOS 7.2.0.2 (Business Edition) Xcode ver 5.1