http://www.cocoanetics.com/2011/08/adding-fading-gradients-to-uitableview/

gistfile1.m

- (void)viewWillAppear:(BOOL)animated
{
    [super viewWillAppear:animated];
 
    if (!maskLayer)
    {
        maskLayer = [CAGradientLayer layer];
 
        CGColorRef outerColor = [UIColor colorWithWhite:1.0 alpha:1.0].CGColor;
        CGColorRef innerColor = [UIColor colorWithWhite:1.0 alpha:0.0].CGColor;
 
        maskLayer.colors = [NSArray arrayWithObjects:(id)outerColor, 
                            (id)innerColor, (id)innerColor, (id)outerColor, nil];
        maskLayer.locations = [NSArray arrayWithObjects:[NSNumber numberWithFloat:0.0], 
                            [NSNumber numberWithFloat:0.2], 
                            [NSNumber numberWithFloat:0.8], 
                            [NSNumber numberWithFloat:1.0], nil];
 
        maskLayer.bounds = CGRectMake(0, 0,
                            self.tableView.frame.size.width,
                            self.tableView.frame.size.height);
        maskLayer.anchorPoint = CGPointZero;
 
        [self.view.layer addSublayer:maskLayer];
    }
}

- (void)scrollViewDidScroll:(UIScrollView *)scrollView
{
    [CATransaction begin];
    [CATransaction setDisableActions:YES];
    maskLayer.position = CGPointMake(0, scrollView.contentOffset.y);
    [CATransaction commit];
}