Get UIScrollView to scroll to the top

UPDATE FOR iOS 7

[self.scrollView setContentOffset:
    CGPointMake(0, -self.scrollView.contentInset.top) animated:YES];

ORIGINAL

[self.scrollView setContentOffset:CGPointZero animated:YES];

or if you want to preserve the horizontal scroll position and just reset the vertical position:

[self.scrollView setContentOffset:CGPointMake(self.scrollView.contentOffset.x, 0)
    animated:YES];