Apple has added a tiny line over the tabBar in iOS 7 which is supposed to work as a shadow or fade between the tabBar and the UI
In my case I also needed to set a different shadow, in the end the only thing that worked while also setting a custom shadow was to add a single-point high UIView 1 point above the tab bar:
UIView *whiteLine = [[UIView alloc] initWithFrame:CGRectMake(0.0, -1.0, self.tabBar.frame.size.width, 1.0)];
whiteLine.backgroundColor = [UIColor whiteColor];
[self.tabBar addSubview:whiteLine];