That's an easy one:
[aView convertPoint:localPosition toView:nil];
... converts a point in local coordinate space to window coordinates. You can use this method to calculate a view's origin in window space like this:
[aView.superview convertPoint:aView.frame.origin toView:nil];
2014 Edit: Looking at the popularity of Matt__C's comment it seems reasonable to point out that the coordinates...
- don't change when rotating the device.
- always have their origin in the top left corner of the unrotated screen.
- are window coordinates: The coordinate system ist defined by the bounds of the window. The screen's and device coordinate systems are different and should not be mixed up with window coordinates.