If you are creating a sheet (beginSheet), but the sheet doesn't want to attach itself to the window even though everything is set up properly (it floats in the middle of the app instead of dropping down from behind the title bar), make sure that "Visible At Launch" is NOT checked in the NIB. Spent hours chasing this stupid problem down. Bloody hell.
This can also happen if you try to access IBOutlets before the window appears. The sheet still appears but is floating.
This is xcode circa 3.2.5 but I've seen it on earlier versions as well.